Exhibit 99.1

Horizon Pharma plc

UNAUDITED PRO FORMA COMBINED FINANCIAL INFORMATION

Horizon Pharma plc Unaudited Pro Forma Combined Financial Statements

The following unaudited pro forma combined financial information is presented to illustrate the estimated effects of the following transactions (collectively, the “Pro Forma Transactions”): (i) the merger of Ghrian Acquisition Inc. (“Purchaser”), a wholly owned subsidiary of Horizon Pharma, Inc. (“HPI”), with and into Hyperion Therapeutics, Inc. (“Hyperion”), with Hyperion continuing as the surviving corporation and as an indirect wholly owned subsidiary of HPI (the “Acquisition”), (ii) the April 2015 equity offering by Horizon Pharma Public Limited Company (the “Company”) of 17,652,500 ordinary shares (the “Equity Offering”), (iii) the Company’s March 2015 private placement of $400.0 million in aggregate principal amount of 2.5% Exchangeable Senior Notes due 2022 (the “Exchangeable Senior Notes”), (iv) the Company’s April 2015 private placement of $475.0 million in aggregate principal amount of 6.625% Senior Notes due 2023 (the “Senior Unsecured Notes”), (v) the Company’s entry into a term loan facility and borrowing of $400.0 million under the facility on May 7, 2015 (the “Senior Secured Term Loan”), (vi) the merger of the businesses of HPI and Vidara Therapeutics International Public Limited Company (“Vidara”) on September 19, 2014 (the “Vidara Merger”), and (vii) the Company’s entry into a credit facility and borrowing of $300.0 million under the facility in connection with the Vidara Merger (the “Prior Credit Facility”).

The historical pro forma combined balance sheet information as of March 31, 2015 is based upon and derived from the historical financial information of the Company and Hyperion and gives effect to the Pro Forma Transactions as if such transactions had occurred on March 31, 2015 except for the Vidara Merger and the Prior Credit Facility, which are already reflected in the Company’s historical balance sheet as of March 31, 2015. The unaudited pro forma combined statements of operations for the year ended December 31, 2014 and the three months ended March 31, 2015 are based upon and derived from the historical financial information of the Company, Hyperion and Vidara and give effect to the Pro Forma Transactions as if they occurred on January 1, 2014.

The Acquisition is being accounted for as a business combination using the acquisition method of accounting under the provisions of Accounting Standards Codification (“ASC”) 805, “Business Combinations” (“ASC 805”). The Vidara Merger has been accounted for as a reverse acquisition under the acquisition method of accounting under the provisions of ASC 805.

The pro forma adjustments are preliminary and are based upon the best available information and certain assumptions, as described in the accompanying notes to the unaudited pro forma combined financial information, that the Company’s management believes are reasonable under the circumstances and which are described in the accompanying notes to the unaudited pro forma combined financial information.

Under ASC 805, assets acquired and liabilities assumed are generally recorded at their acquisition date fair value. The preliminary fair value of identifiable tangible assets acquired and liabilities assumed from the Acquisition are based on their carrying value as of March 31, 2015. The preliminary fair value of intangible assets and contingent royalty liabilities assumed has been estimated using the income approach through a discounted cash flow analysis. After the closing of the Acquisition, the Company updated the preliminary purchase price allocation based upon the fair market values as of the closing date of the Acquisition. Any excess of the purchase price over the fair value of identified assets acquired and liabilities assumed was recognized as goodwill. Significant judgment was required in determining the estimated fair values of developed technology intangible assets and certain other assets and liabilities. Such a valuation required estimates and assumptions including, but not limited to, estimating future cash flows and direct costs in addition to developing the appropriate discount rates and current market profit margins. The Company’s management believes the fair values recognized for the assets acquired and the liabilities assumed are based on reasonable estimates and assumptions. Accordingly, the unaudited pro forma purchase price adjustments are preliminary and are subject to further adjustments as additional information becomes available and as additional analyses are performed, and such further adjustments may be material.

The unaudited pro forma combined financial information has been prepared by the Company’s management in accordance with Article 11 of Regulation S-X for illustrative purposes only and is not necessarily indicative of the combined financial position or results of operations that would have been realized had the transactions been completed as of the dates indicated, nor is it meant to be indicative of any anticipated combined financial position or future results of operations that the Company will experience following the completion of the Pro Forma Transactions. In addition, the accompanying unaudited pro forma combined statement of operations do not include any pro forma adjustments to reflect expected cost savings or restructuring actions which may be achievable or the impact of any non-recurring activity and one-time transaction related costs.

Certain financial information of Hyperion and Vidara, as presented in their respective consolidated financial statements, has been reclassified to conform to the historical presentation in the Company’s consolidated financial statements for purposes of preparation of the unaudited pro forma combined financial information. See Notes 4 and 5 for additional information on the reclassifications that were made to derive the “Historical Hyperion (after conforming reclassifications)” and “Historical Vidara (after conforming reclassifications)” columns in the unaudited pro forma combined financial statements.

The unaudited pro forma combined financial statements, including the notes thereto, should be read in conjunction with the (1) the historical unaudited consolidated financial statements of the Company included in its Quarterly Report on Form 10-Q for the quarter ended March 31, 2015, (2) the historical consolidated financial statements of the Company included in its Annual Report on Form 10-K for the year ended December 31, 2014, (3) the historical unaudited consolidated financial statements of Hyperion included in its Quarterly Report on Form 10-Q for the quarter ended March 31, 2015, (4) the historical consolidated financial statements of Hyperion included in its Annual Report on Form 10-K for the year ended December 31, 2014, and (5) the historical combined financial statements of Vidara included in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2014.

Unaudited Pro Forma Combined Balance Sheet

As of March 31, 2015

(In thousands, except for share data)

See the accompanying notes to the unaudited pro forma combined financial information, which are an integral part of these pro forma financial statements.

Unaudited Pro Forma Combined Statement of Operations

For the Three Months Ended March 31, 2015

(In thousands, except for share and per share data)

See the accompanying notes to the unaudited pro forma combined financial information, which are an integral part of these pro forma financial statements.

Unaudited Pro Forma Combined Statement of Operations

For the Year Ended December 31, 2014

(In thousands, except for share and per share data)

See the accompanying notes to the unaudited pro forma combined financial information, which are an integral part of these pro forma financial statements.

The Acquisition: On March 29, 2015, HPI entered into an Agreement and Plan of Merger with Purchaser and Hyperion, pursuant to which, on May 7, 2015, Purchaser merged with and into Hyperion, in accordance with Section 251(h) of the General Corporation Law of the State of Delaware, with Hyperion continuing as the surviving corporation and as an indirect wholly owned subsidiary of HPI.

Exchangeable Senior Notes. On March 13, 2015, Horizon Pharma Investment Limited, a wholly-owned subsidiary of the Company, completed its private placement of $400.0 million in aggregate principal amount of Exchangeable Senior Notes to several investment banks acting as initial purchasers who subsequently resold the Exchangeable Senior Notes to qualified institutional buyers as defined in Rule 144A under the Securities Act of 1933, as amended (the “Securities Act”).

Equity Offering: On April 21, 2015, the Company closed its underwritten public offering of 17,652,500 of its ordinary shares at a price to the public of $28.25 per share. The net proceeds to the Company from such offering were approximately $475.2 million, after deducting underwriting discounts and other offering expenses payable by the Company.

Senior Unsecured Notes: On April 29, 2015, Horizon Pharma Financing Inc., a wholly-owned subsidiary of the Company, completed a private placement of $475.0 million in aggregate principal amount of Senior Unsecured Notes to certain investment banks acting as initial purchasers who subsequently resold the Senior Unsecured Notes to qualified institutional buyers as defined in Rule 144A under the Securities Act and in offshore transactions to non-U.S. persons in reliance on Regulation S under the Securities Act.

Senior Secured Term Loan: On May 7, 2015, HPI, the Company and certain of its subsidiaries entered into a credit agreement (the “Credit Agreement”) with Citibank, N.A., as administrative and collateral agent, and the lenders from time to time party thereto providing for (i) a five-year $400.0 million term loan facility (the “Senior Secured Term Loan”); (ii) an uncommitted accordion facility subject to the satisfaction of certain financial and other conditions; and (iii) one or more uncommitted refinancing loan facilities with respect to loans thereunder. The initial borrower under the Senior Secured Term Loan is HPI. The Credit Agreement allows for the Company and certain other subsidiaries of the Company to become borrowers under the accordion or refinancing facilities. Loans under the Senior Secured Term Loan bear interest, at each borrower’s option, at a rate equal to either the London Inter-Bank Offer Rate (“LIBOR”), plus an applicable margin of 3.5% per year (subject to a 1.0% LIBOR floor), or the prime lending rate, plus an applicable margin equal to 4.5% per year. The Company borrowed the full $400.0 million available under the Senior Secured Term Loan on May 7, 2015 as a LIBOR-based borrowing.

Vidara Merger: On September 19, 2014, the Company acquired Vidara for $601.4 million, comprised of the $387.8 million market value of the 31,350,000 ordinary shares of the Company that were held by prior Vidara shareholders immediately following the closing of the Vidara Merger plus the cash consideration of $213.6 million.

The historical consolidated financial information of the Company has been adjusted in the accompanying unaudited pro forma combined financial information to give effect to pro forma events that are (i) directly attributable to the Pro Forma Transactions, (ii) factually supportable, and (iii) with respect to the unaudited pro forma combined statements of operations, expected to have a continuing impact on the results of operations.

The Acquisition is being accounted for as a business combination using the acquisition method of accounting under the provisions of ASC 805. The unaudited pro forma combined financial information was prepared using the acquisition method of accounting, which requires, among other things, that assets acquired and liabilities assumed in a business combination be recognized at their fair values as of the acquisition date. The adjustments to reflect the acquisition method of accounting are preliminary and are based upon the best available information and certain assumptions which management believes are reasonable under the circumstances.

The acquisition method of accounting uses the fair value concepts defined in ASC 820, “Fair Value Measurement” (“ASC 820”), as “the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.” This is an exit price concept for the valuation of an asset or liability. Market participants are assumed to be buyers or sellers in the most advantageous market for the asset or liability. Fair value measurement for an asset assumes the highest and best use by these market participants. Fair value measurements can be highly subjective and it is possible the application of reasonable judgment could develop different assumptions resulting in a range of alternative estimates using the same facts and circumstances.

Cash and cash equivalents, investments, and other tangible assets and liabilities: The carrying amounts of tangible assets and liabilities were valued at their respective carrying amounts as management believes that these amounts approximate their current fair values.

Inventories: Inventories acquired included raw materials and finished goods. Inventories are recorded at their current fair values. Fair value of finished goods has been determined based on the estimated selling price, net of selling costs and a margin on the selling costs. Fair value of raw materials has been estimated to equal the replacement cost.

Developed technology: Developed technology intangible assets reflect the estimated value of Hyperion’s rights to its currently marketed products, RAVICTI and BUPHENYL. The fair value of developed technology was determined using an income approach. The income approach explicitly recognizes that the fair value of an asset is premised upon the expected receipt of future economic benefits such as earnings and cash inflows based on current sales projections and estimated direct costs for Hyperion’s product line. Indications of value are developed by discounting these benefits to their present worth at a discount rate that reflects the current return requirements of the market. The fair value of the RAVICTI and BUPHENYL developed technologies will be capitalized as of the Acquisition date and subsequently amortized over the 11 and 7 years, respectively, which are the periods in which over 90% of the estimated cash flows are expected to be realized.

Goodwill: Goodwill represents the excess of the preliminary acquisition consideration over the estimated fair values of net assets acquired.

Deferred tax assets and liabilities: Deferred tax assets and liabilities arise from acquisition accounting adjustments where book values of certain assets and liabilities differ from their tax bases. Deferred tax assets and liabilities are recorded at the currently enacted rates which will be in effect at the time when the temporary differences are expected to reverse in the country where the underlying assets and liabilities are located. Hyperion’s developed technology as of the acquisition date was located primarily in the United States where a U.S. tax rate of 39% is being utilized and a deferred tax liability is recorded. Upon consummation of the Acquisition, Hyperion became a member of the Company’s U.S. tax consolidation group. As such, its tax assets and liabilities need to be considered in determining the appropriate amount (if any) of valuation allowance that should be recognized in assessing the realizability of the group’s deferred tax assets. The Hyperion acquisition adjustments resulted in the recording of significant net deferred tax liabilities. Per ASC 740, “Accounting for Uncertainty in Income Taxes”, future reversals of existing taxable temporary differences must be considered in determining the amount of valuation allowance to record. As of March 31, 2015, the Company had a significant U.S. federal and state valuation allowance which must be reduced to reflect the addition of Hyperion’s deferred tax liabilities that will provide taxable temporary differences that will be realized within the carryforward period of the group’s available net operating losses and other deferred tax assets. Accordingly, the result will be to recognize the release of most of the existing U.S. federal and state valuation allowance of approximately $105.0 million. However, this was not reflected as a pro forma adjustment as this is a one-time transaction cost.

Pre-existing contingencies: The Company has identified a contingent liability potentially payable under previously existing royalty and licensing agreements. The initial fair value of this liability was determined using a discounted cash flow analysis incorporating the estimated future cash flows of royalty payments based on future sales. The liability will be periodically assessed based on events and circumstances related to the underlying milestones, and any change will be recorded in the Company’s consolidated statement of operations.

The preliminary determination of the fair value of the acquired net assets, assuming the Acquisition had closed on January 1, 2014 is as follows (in thousands, except share and per share data):

Following the Acquisition, the Company is conducting a review of accounting policies of Hyperion in an effort to determine if differences in accounting policies require restatement or reclassification of results of operations or reclassification of assets or liabilities to conform to the Company’s accounting policies and classifications. As a result of that review, the Company may identify differences among the accounting policies of the Company and Hyperion that, when conformed, could have a material impact on this unaudited pro forma combined financial information. During the preparation of this unaudited pro forma combined financial information, the Company was not aware of any material differences between accounting policies of the Company and Hyperion, except for certain reclassifications necessary to conform to the Company’s financial presentation, and accordingly, this unaudited pro forma combined financial information does not assume any material differences in accounting policies among the Company and Hyperion.

Financial information of Hyperion in the “Historical Hyperion (after conforming reclassifications)” column in the unaudited pro forma combined balance sheet represents the historical consolidated balance sheet of Hyperion as of March 31, 2015. Financial information presented in the “Historical Hyperion (after conforming reclassifications)” column in the unaudited pro forma combined statement of operations represents the historical consolidated statement of earnings of Hyperion for the three months ended March 31, 2015 and the year ended December 31, 2014, which includes the operations from Andromeda Biotech Ltd. (“Andromeda”) since June 12, 2014, the date of acquisition. The Company does not believe that the operating results for Andromeda prior to its acquisition by Hyperion on June 12, 2014 are material to the combined entity and as such have not been included in the unaudited pro forma combined statement of operations. Such financial information has been reclassified or classified to conform to the historical presentation in the Company’s consolidated financial statements as set forth below. Unless otherwise indicated, defined line items included in the footnotes have the meanings given to them in the historical financial statements of Hyperion.

Reclassifications in Hyperion’s unaudited pro forma combined balance sheet as of March 31, 2015 are as follows (in thousands, except share data):

Reclassifications in Hyperion’s unaudited pro forma combined statement of operations for the three months ended March 31, 2015 are as follows (in thousands):

Reclassifications in Hyperion’s unaudited pro forma combined statement of operations for the year ended December 31, 2014 are as follows (in thousands):

Financial information presented in the “Historical Vidara (after conforming reclassifications)” column in the unaudited pro forma combined statement of operations for the year ended December 31, 2014 represents Vidara results of operations as a stand-alone entity for the period from January 1, 2014 to September 18, 2014, which were derived from its unaudited combined financial statements included in its Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2014 and the stub period from July 1, 2014 to September 18, 2014. Financial information of Vidara subsequent to September 18, 2014 is included in the statement of operations of the Company for the year ended December 31, 2014.

Reclassifications and classifications in Vidara’s unaudited pro forma combined statement of operations for the year ended December 31, 2014 are as follows (in thousands):

The Senior Secured Term Loan and the Senior Unsecured Notes include the following adjustments (in thousands):

(1) Excludes Hyperion’s $18,192 of debt, $7,923 of intangible assets, $361 of deferred rent, $2,586 of Andromeda deferred gain and $400 of employee receivables, which were not assumed by the Company as part of the Acquisition.

Pro forma earnings per share gives effect to the issuance of 17,652,500 ordinary shares from the Equity Offering and the pro forma adjustments.