Voters on Friday will be asked whether or not to add an article to the Irish constitution saying: "Marriage may be contracted in accordance with law by two persons without distinction as to their sex."
The latest polls show the "Yes" side in the lead. Volunteers for and against gay marriage have been canvassing door to door in recent weeks and billboards have appeared appealing for votes, as a colourful "Yes" bus makes its way around the country.
But a "Yes" victory is by no means certain and there is concern among proponents about whether "Yes" supporters will come out and cast their ballots.
"We will ensure that people will be treated equally, no matter who they love," Prime Minister Enda Kenny has said.
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The move is opposed by the Catholic Church, whose influence has waned in Ireland amid growing secularisation and after a wave of child sex abuse scandals that badly discredited the hierarchy.
An Irish Times/Ipsos MRBI poll published yesterday showed 70 per cent support for the "Yes" side and 30 per cent for the "No". The same pollsters in March had given the "Yes" side 78 per cent.
There is a clear divide in the polls, with many older people and rural residents intending to vote "No".
Eighteen countries around the world have so far legalised gay marriage or are about to do so, including 13 in Europe. Across the border in Northern Ireland, gay marriage is banned even though it is legal in the rest of Britain.
The constitutional referendum in Ireland would have to be followed by specific legislation passed in parliament.