Who can initiate an optional referendum at the national level?

Constitution of Ireland

(Adopted on December 1937, last amended June 2004)

http://?www.?constitution.?org/?cons/?ireland/?constitution_?ireland-?en.?pdf

Article 27.(Optional referendum)        

This Article applies to any Bill, other than a Bill expressed to be a Bill containing a proposal for the amendment of this Constitution, which shall have been deemed, by virtue of Article 23 hereof, to have been passed by both Houses of the Oireachtas.

5.1) In every case in which the President decides that a Bill the subject of a petition under this Article contains a proposal of such national importance that the will of the people thereon ought to be ascertained, he shall inform the Taoiseach and the Chairman of each House of the Oireachtas accordingly in writing under his hand and Seal and shall decline to sign and promulgate such Bill as a law unless and until the proposal shall have been approved either :

i) by the people at a Referendum in accordance with the provisions of section 2 of Article 47 of this Constitution within a period of eighteen months from the date of the President’s decision, or

ii) by a resolution of Dáil Éireann passed within the said period after a dissolution and re-assembly of Dáil Éireann. 

http://?www.?environ.?ie/?en/?Publications/?LocalGovernment/?Voting/?FileDownLoad,1893,en.?pdf

Articles 27 and 47 of the Constitution provide for a referendum on a proposal other than a proposal to amend the Constitution (referred to in law as an "ordinary referendum"). An ordinary referendum may take place when the President, on receipt of a joint petition from a majority of the members of the Seanad and not less than one third of the members of the Dáil and following consultation with the Council of State, decides that the Bill contains a proposal of such national importance that the will of the people thereon ought to be ascertained before the measure becomes law.