Ireland has gone to the polls to vote on whether same-sex marriage should be legal, in a referendum that has exposed sharp divisions between communities in this traditionally Catholic nation.

MORE: Louisa Wall - Marriage equality in Ireland

It is the first country in the world to hold such a referendum.

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.

"A lot of my gay friends want to get married. I feel it's a simple matter of equality," said Eoghan Bonass, 35, voting in a polling station in a south Dublin suburb on Friday.

"This referendum has touched a chord. I've noticed there's far more awareness about this one than previous ones," he said.

Rachael Stanley, 60, said she voted "No" and felt "strongly about it".

"This is about children. It's far too radical a step. I want to protect marriage and the stability of children," she said.

"I hope I don't get tarred and feathered for saying that," she added.

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

Referenda in Croatia and Slovenia both resulted in "No" votes, although in Slovenia, parliament went ahead and approved gay marriage in March.

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

Ireland would be the 19th country in the world to legalise gay marriage and the 14th in Europe.

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

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.

"This burden and pressure that's been on (gay couples), living in the shadows - that can be removed on Friday by voting 'Yes'," Kenny said.

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.

"I think you can have equality while recognising difference. For me, the fundamental thing is marriage and a family are about the complimentary gifts of a man and a woman, a mother and a father."

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.

Voter Jodie Boylan, 41, said she was too excited to sleep last night.

"It's a historic day for Ireland. After all we've been through with the Church and everything else what a message it would be to send that we are all equal and can be treated the same."

"I really hope it passes," she said.

Polls close at 9am (NZT), with the final result not expected for some hours.