Quorum requirements for a referendum to be valid

The procedure at an ordinary referendum is similar to that in relation to a Constitutional referendum except that the proposal is held to have been vetoed by the people if the majority of the votes are cast against the proposal and such votes represent at least one-third of the presidential electors on the register of electors. No ordinary referendum has been held in the State to date.

Constitutionof Ireland

(Adoptedon December 1937, last amended June 2004)

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

Article47.

2.1?Every proposal, other than a proposal to amend the Constitution, which issubmitted by Referendum to the decision of the people shall be held to havebeen vetoed by the people if a majority of the votes cast at such Referendumshall have been cast against its enactment into law and if the votes so castagainst its enactment into law shall have amounted to not less than thirty-threeand one-third per cent of the voters on the register. 

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

Mandatory referendum - no quorums requirements. Optional referendum - 1/3 of of the voters on the register.