UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

Amendment No. 1

(Mark One)

For the fiscal year ended December 31, 2014

or

For the transition period from             to            

Commission File Number 001-35238

HORIZON PHARMA PUBLIC LIMITED COMPANY

(Exact name of Registrant as specified in its charter)

011 353 1 772 2100

(Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act:

None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  x    No  ¨.

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x.

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Website, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of the registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company.

Indicate by check mark whether the registrant is a shell company (as defined by Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

The aggregate market value of the registrant’s voting ordinary shares held by non-affiliates of the registrant, based upon the $15.82 per share closing sale price of the registrant’s ordinary shares on June 30, 2014 (the last business day of the registrant’s most recently completed second quarter), was approximately $1.0 billion. Solely for purposes of this calculation, the registrant’s directors and executive officers and holders of 10% or more of the registrant’s outstanding ordinary shares have been assumed to be affiliates and an aggregate of 9,164,811 shares of the registrant’s voting ordinary shares held by such persons on June 30, 2014 are not included in this calculation.

As of February 20, 2015, the registrant had outstanding 125,100,210 ordinary shares.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive Proxy Statement for the registrant’s 2015 Annual Meeting of Shareholders are incorporated by reference into Part III of Annual Report on this Form 10-K.

EXPLANATORY NOTE

The sole purpose of this Amendment No. 1 to our Annual Report on Form 10-K for the year ended December 31, 2014, originally filed with the Securities and Exchange Commission on February 27, 2015 (the “Original Filing”) is to correct a clerical error that resulted in the filing of an incorrect version of Exhibit 101 to the Form 10-K, which contains the XBRL (eXtensible Business Reporting Language) Interactive Data File for the financial statements and notes included in Part IV, Item 15 of the Form 10-K.

No other changes have been made to the Form 10-K. This Form 10-K/A does not does not modify or update any previously reported financial statements or other disclosures in the Original Filing. Accordingly, this Amendment No. 1 should be read in conjunction with the Original Form 10-K, which continues to speak as of the original filing date of the Original Filing.

INDEX TO EXHIBITS

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this Annual Report on Form 10-K/A to be signed on its behalf by the undersigned, thereunto duly authorized.

Dated: February 27, 2015