Ireland has voted to allow gay marriage, with all constituencies bar one saying yes to constitutional reform

Ireland will legalise gay marriage after voters overwhelmingly backed allowing same-sex unions in a historic referendum.

Celebrations spilled out into Dublin city centre as it became clear that the campaign to extend the right to marry to same-sex couples was heading for a landslide victory.

With more than half of the results in, all constituencies bar one have so far voted in favour of gay marriage, with more than 62 per cent backing reform.

Ireland's equalities minister Aodhán Ó Ríordáin lost his composure as the result became clear, tweeting: 'Ireland hasn't just said "Yes"... Ireland has said: "F*** YEAAHHHH"'.

With 39 constituencies so far returning official results, Ireland is on course to pass gay marriage rights reform more than 62 per cent in favour.

All but one region - Roscommon South Leitrim - have backed same-sex unions at this stage, with over a million voting for and 640,000 against across Ireland.

The biggest support was recorded in Dublin Central with a 72 per cent majority for the Yes side, but there were closer results in some parts of the nation.

Gay rights activists were seen hugging and celebrating as Ireland appeared to be on the verge of becoming the first country in the world to introduce same-sex marriage by popular vote. 

In Dublin the result was expected to be a landslide in favour of gay marriage while in many other parts of the country the constitutional amendment was expected to be passed by about two to one.

Equalities minister Mr Ó Ríordáin, following his emotional tweet, said: 'This has really touched a nerve in Ireland. It's a very strong message to every LGBT (lesbian, gay, bisexual, and transgender) young person in Ireland and every LGBT young person in the world.'

Speaking on Sky News, Sinn Fein leader Gerry Adams said this was 'a huge day for equality'. 

He said: 'I also think that given that the government parties were pressing quite rightly for equality in this issue then we need equality in other issues - we need equality in social issues, economic issues we need everything to be equality. So this is a hugely important day for the LGBT community and everybody else.' 

Ireland's deputy prime minister, Labour Party leader Joan Burton, said Ireland was becoming 'a rainbow nation with a huge amount of diversity'. 

Leo Varadkar, Ireland's health minister and first openly gay cabinet member, said it was a special day.

'It seems to me that the Irish people had their minds made up on this some time ago,' he said.

Senator David Norris, one of the key figures in having homosexuality decriminalized in the 1990s, said: 'I believe that by the end of today gay people will be equal in this country. I think it's wonderful.

'It's a little bit late for me. As I said the other day I've spent so much time pushing the boat out that I forgot to jump on and now it's out beyond the harbour on the high seas, but it's very nice to look at.' 

Pat Carey, a former government minister who came out as gay earlier this year aged 68, welled up saying: 'It's down to the brave, ordinary people of Ireland who took their courage in their hands.' 

Young people are said to have turned out in unprecedented numbers to cast their vote, with an official result due this afternoon. 

Taoiseach Enda Kenny said there was a movement of young people who were firmly behind the reforms.

'I think from a young person's perspective, particularly for those who travelled from wherever to wherever to put a simple mark on a paper, shows the value of the issue and the importance of politics,' he said.  

John Murray, director of the Iona Institute religious think-tank - one of the leading groups fighting the reform - admitted defeat this morning.

'Obviously I'd be disappointed in that. We did our best to fight a good campaign and if having run that we haven't won it, we did our best,' he said.

'Everyone is saying it is Yes and I'm not going to argue with that.'

Results from the constituencies will be fed back to the count headquarters in Dublin Castle, where young people draped in rainbow-coloured flags sat in circles awaiting the result, talking to curious tourists.

The Irish government, which lobbied for a Yes vote, has opened the building to campaigners.

Irish voters were asked whether they support or reject a change to the country's constitution, so it would read: 'Marriage may be contracted in accordance with law by two persons without distinction as to their sex.'

The constitution's definition of family will not be changed, including a section that states a woman's place is at home. 

Other countries have held referendums on gay marriage, including Croatia, Slovakia and Slovenia, where same-sex unions were voted against by the electorate.

Ireland decriminalized homosexuality just 22 years ago in 1993, while the UK made gay sex legal in 1967.

The first gay marriages in Britain took place last year but they are still outlawed in Northern Ireland.

In a separate referendum also being held today, voters are being asked their views on proposals to reduce the age limit on who can stand for the presidency from 35 to 21.