POWER OF ATTORNEY

        KNOW ALL BY THESE PRESENTS, that the undersigned hereby constitutes and
appoints A. W. Homan, Karin L. Walker and Kevin J. Hickey, and their respective
successors, and each of them the undersigned's true and lawful attorneys-in-fact
and agents with full power of substitution, signing singly, for the undersigned
and in the undersigned's name, place and stead, in any and all capacities, to:

    1.  prepare, execute in the undersigned's name and on the undersigned's
        behalf, and submit to the U.S. Securities and Exchange Commission (the
        "SEC") a Form ID, including amendments thereto, and any other documents
        necessary or appropriate to obtain codes and passwords enabling the
        undersigned to make electronic filings with the SEC of reports required
        by Section 16(a) of the Securities Exchange Act of 1934 (as amended, the
        "Exchange Act") or any rule or regulation of the SEC;

    2.  complete, execute and timely file for and on behalf of the undersigned,
        in the undersigned's capacity as an officer and/or director of Prothena
        Corporation plc (the "Company"), Forms 3, 4 and 5, any other forms, and
        any amendments thereto in accordance with Section 16(a) of the Exchange
        Act and the rules and regulations thereunder; and

    3.  take any other action of any type whatsoever in connection with the
        foregoing which, in the opinion of such attorney-in-fact, may be of
        benefit to, in the best interest of, or legally required by, the
        undersigned, it being understood that the documents executed by such
        attorney-in-fact on behalf of the undersigned pursuant to this Power of
        Attorney shall be in such form and shall contain such terms and
        conditions as such attorney-in-fact may approve in such
        attorney-in-fact's discretion.

        The undersigned hereby grants to each such attorney-in-fact full power
and authority to do and perform any and every act and thing whatsoever
requisite, necessary or proper to be done in the exercise of any of the rights
and powers herein granted, as fully to all intents and purposes as the
undersigned might or could do if personally present, with full power of
substitution or revocation, hereby ratifying and confirming all that such
attorney-in-fact, or such attorney-in-fact's substitute or substitutes, shall
lawfully do or cause to be done by virtue of this power of attorney and the
rights and powers herein granted. The undersigned acknowledges that the
foregoing attorneys-in-fact, in serving in such capacity at the request of the
undersigned, are not assuming, nor is the Company assuming, any of the
undersigned's responsibilities to comply with Section 16 of the Exchange Act.

        The undersigned agrees that each such attorney-in-fact herein may rely
entirely on information furnished orally or in writing by the undersigned to
such attorney-in-fact. The undersigned also agrees to indemnify and hold
harmless the Company and each such attorney-in-fact against any losses, claims,
damages or liabilities (or actions in these respects) that arise out of or are
based upon any untrue statements or omission of necessary facts in the
information provided by the undersigned to such attorney-in fact for purposes of
executing, acknowledging, delivering or filing Forms 3, 4 or 5, Form ID or any
other form (including amendments thereto) and agrees to reimburse the Company
and such attorney-in-fact for any legal or other expenses reasonably incurred in
connection with investigating or defending against any such loss, claim, damage,
liability or action.

        This Power of Attorney supersedes any power of attorney previously
executed by the undersigned regarding the purposes outlined in the first
paragraph hereof ("Prior Powers of Attorney"), and the authority of the
attorneys-in-fact named in any Prior Powers of Attorney is hereby revoked.

        This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4 and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier (a) revoked by the undersigned in a signed writing delivered to
the foregoing attorneys-in-fact or (b) superseded by a new power of attorney
regarding the purposes outlined in the first paragraph hereof dated as of a
later date.

        IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 8th day of May, 2017.

                                        Signature:   /s/ Sarah B. Noonberg
                                                   -----------------------------
                                                   Name: Sarah B. Noonberg