Ireland has become the first country in the world to approve gay marriage by popular vote as crowds cheered in Dublin in a spectacular setback for the once all-powerful Catholic Church.

With ballots in 40 out of 43 constituencies counted, the "Yes" vote was ahead with 62.3 per cent, official figures showed on Saturday, and public broadcaster RTE said the result from the ongoing count was now certain.

Hundreds of "Yes" supporters gathered in the grounds of Dublin Castle cheered and waved rainbow flags as the results came through.

Legalising gay marriage is a seismic change in Ireland, where the Church remains a powerful force.

Homosexuality was illegal until 1993 and divorce until 1996, and abortion is banned except where the mother's life is in danger.

The Archbishop of Dublin Diarmuid Martin, Ireland's top Catholic leader, told RTE that the Church now needed a "reality check".

"I think the Church needs to do a reality check right across the board... Have we drifted away completely from young people?" he said.

"It's a social revolution that didn't begin today," said Martin, who had called for a "No" vote arguing that gay rights should be respected "without changing the definition of marriage".

Health Minister Leo Varadkar, who became Ireland's first openly gay cabinet minister when he came out this year and is tipped as a future leader, also said the "Yes" campaign had been a "social revolution".

The referendum has pitched traditionalists including the Catholic Church against those in favour of gay marriage, including Ireland's Prime Minister Enda Kenny, a Catholic who told voters there was "nothing to fear".

All Ireland's main political parties supported amending the constitutional definition of marriage.

A string of Irish celebrities have also backed the "Yes" campaign including singers Bono and Sinead O'Connor plus actor Colin Farrell.

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

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

Parliament first has to pass specific legislation and is expected to draw up the bill this week. The first same-sex weddings could then take place in Ireland towards the end of the year.