Voters in the Republic are expected to decide on May 22 to accept gay marriage in what campaigners have called a monumental referendum.
Opinion polls suggest a majority are in favour of the reform - meaning Northern Ireland could be the only part of the UK or Ireland where gay marriage is not legal in just three months' time.
Rights groups supporting the proposed reform said the announcement of the poll date marks a countdown to equality.
Grainne Healy, chairwoman of the Marriage Equality group, said all loving couples should be free to unite using the same commitment.
"Marriage is important to Irish society, it's a secure foundation for committed and loving couples. Everyone should be free to marry on those terms," she said.
Homosexuality was a criminal offence in the Republic until 1993 and in 2011 gay people were given the right to commit to civil partnerships.