UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 8-K/A

Amendment No. 1

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): March 29, 2015

Horizon Pharma Public Limited Company

(Exact name of registrant as specified in its charter)

Connaught House, 1^st Floor, 1 Burlington Road, Dublin 4, Ireland

(Address of principal executive offices)

Registrant’s telephone number, including area code: 011-353-1-772-2100

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Item 1.01. Entry into a Material Definitive Agreement.

On March 31, 2015, Horizon Pharma Public Limited Company (“Horizon”) filed a Current Report on Form 8-K (the “Original Report”). The Original Report included a version of the Agreement and Plan of Merger, dated as of March 29, 2015, by and among Horizon Pharma, Inc. (“Parent”), Ghrian Acquisition Inc. (“Purchaser”) and Hyperion Therapeutics, Inc. (“Target”), which included typographical errors. This Current Report on Form 8-K/A (Amendment No. 1) is being filed solely to replace Exhibit 2.1.

A copy of the corrected Merger Agreement is attached to this Amendment No. 1 as Exhibit 2.1.

Forward-Looking Statements

This current report contains forward-looking statements, including, but not limited to, statements related to Parent’s anticipated acquisition of Target and the timing and benefits thereof, estimated future financial results and performance of RAVICTI and BUPHENYL and Parent’s business as a whole, Parent’s financing plans, the combined company’s strategy, plans, objectives, expectations and intentions, anticipated product portfolio, and other statements that are not historical facts. These forward-looking statements are based on Parent’s current expectations and inherently involve significant risks and uncertainties. Actual results and the timing of events could differ materially from those anticipated in such forward-looking statements as a result of these risks and uncertainties, which include, without limitation, risks related to Parent’s ability to complete the acquisition on the proposed terms and schedule; whether Parent or Target will be able to satisfy their respective closing conditions related to the acquisition; whether sufficient stockholders of Target tender their shares in the acquisition; whether Parent will obtain financing for the transaction on the expected timeline and terms; the outcome of legal proceedings that may be instituted against Target and/or others relating to the acquisition; the possibility that competing offers will be made; risks associated with business combination transactions, such as the risk that the businesses will not be integrated successfully, that such integration may be more difficult, time-consuming or costly than expected or that the expected benefits of the acquisition will not occur; risks related to future opportunities and plans for the combined company, including uncertainty of the expected financial performance and results of the combined company following completion of the proposed acquisition; disruption from the proposed acquisition, making it more difficult to conduct business as usual or maintain relationships with customers, employees or suppliers; and the possibility that if the combined company does not achieve the perceived benefits of the proposed acquisition as rapidly or to the extent anticipated by financial analysts or investors, the market price of Parent’s shares could decline, as well as other risks related to Parent’s and Target’s businesses, including the ability to grow sales and revenues from existing products; competition, including potential generic competition; the ability to protect intellectual property and defend patents; regulatory obligations and oversight; and those risks detailed from time-to-time under the caption “Risk Factors” and elsewhere in Parent’s and Target’s respective SEC filings and reports, including their respective Annual Reports on Form 10-K for the year ended December 31, 2014. The Parent undertakes no duty or obligation to update any forward-looking statements contained in this presentation as a result of new information, future events or changes in its expectations.

Additional Information

The offer to acquire all of the outstanding shares of Target’s common stock, par value $0.0001 per share (the “Offer”) by Parent through Purchaser has not yet commenced, and this communication is neither an offer to purchase nor a solicitation of an offer to sell any shares of the common stock of Target or any other securities. On the commencement date of the Offer, a tender offer statement on Schedule TO, including an offer to purchase, a letter of transmittal and related documents, will be filed with the Securities and Exchange Commission by the Purchaser and a Solicitation/Recommendation Statement on Schedule 14D-9 will be filed with the Securities and Exchange Commission by Target. The offer to purchase shares of Target’s common stock will only be made pursuant to the offer to purchase, the letter of transmittal and related documents filed as a part of the Schedule TO. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ BOTH THE TENDER OFFER STATEMENT AND THE SOLICITATION/RECOMMENDATION STATEMENT REGARDING THE OFFER, AS THEY MAY BE AMENDED FROM TIME TO TIME, WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION. The tender offer statement will be filed with the Securities and Exchange Commission by Purchaser, and the solicitation/recommendation statement will be filed with the Securities and Exchange Commission by Target. Investors and security holders may obtain a free copy of these statements (when available) and other documents filed with the Securities and Exchange Commission at the website maintained by the Securities and Exchange Commission at www.sec.gov or by directing such requests to the Information Agent for the Offer, which will be named in the tender offer statement.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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