Ireland goes to the polls on Friday to vote on whether same-sex marriage should be legal, in a referendum that has exposed sharp divisions in the traditionally Catholic country.

Allowing gay couples to wed would be a seismic change in a country where homosexuality was only decriminalised in 1993, and where abortion remains illegal except where the mother's life is in danger.

"The stories that I've heard over the last number of years from ordinary people, in ordinary jobs, this burden and pressure that's been on them, living in the shadows - that can be removed on Friday by voting 'Yes'," Prime Minister Enda Kenny said this week.

If the move is approved, Ireland would become the first country to make the change following a popular vote.

"We are saying here, in a world first, that the people of Ireland can extend the right of civil marriage to all our citizens," Kenny said.

Currently there are 18 countries who have legalised gay marriage, most of them in Europe, but also including South Africa, New Zealand, Canada, Brazil and Argentina.

Across the border in Northern Ireland, gay marriage is banned even though it is legal in the rest of Britain.

All of Ireland's main political parties, including conservatives, support amending the constitutional definition of marriage, and the latest polls put their camp in the lead.

Three opinion polls last weekend showed support for same-sex marriage ranging from 53 to 69 per cent, while the `No' vote is hovering between 24 and 26 per cent.

The `Yes' side has also been boosted by the support of sports, music and film stars including Irish Hollywood A-lister Colin Farrell and U2 frontman Bono.

But the result is by no means certain - the Catholic Church has campaigned strongly for a "No" vote, and many older and rural voters agree with the clergy.

"My voting 'No' is not a vote against gay and lesbian people, it's against changing the definition of marriage," the archbishop of Dublin, Diarmuid Martin, told RTE state television on Wednesday.

The majority of Irish people identify themselves as Catholic, but the Church's influence has waned amid growing secularisation and after a wave of child sex abuse scandals that badly discredited the hierarchy.

Voters 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."