Irish voters have written themselves into the history books by voting for gay marriage rights with a near two to one majority.

With more than a quarter of the results fed in to cheering crowds at Dublin Castle, the Republic was in line to become the first country in the world to back the reform by popular vote.

Official confirmation of the scale of the victory is not expected until after 5pm but a pattern of results from the 43 constituencies showed about 65% of the electorate backed giving same-sex couples the right to marry.

Key campaign groups fighting the reform conceded defeat before any formal declarations.

The referendum now paves the way for the necessary laws to be passed by the summer and potentially opens the door for the first gay marriage ceremonies to be held by the end of the year.

Early on, the reform was most popular in Dublin South West with a near 71% backing.

But in Taoiseach Enda Kenny's own back yard, Co Mayo, the contest was much tighter with only 52% of voters in favour.

It is only 22 years since Ireland decriminalised homosexuality.

Voters were asked one simple, specific question on whether to amend Article 41 of the 1937 Constitution by adding a new clause to a section titled The Family.

It asked them to support or reject a change to the 78-year-old document which reads: "Marriage may be contracted in accordance with law by two persons without distinction as to their sex."

It does not suggest any change to the definition of the family or remove any outdated references in the section, including those that state a woman's place is at home.

If passed, it would be the 34th amendment to the constitution but, regardless of the result, the campaign will rank with other hotly contested issues such as divorce and abortion.

The only blot on the landscape came with the result from the constituency of Roscommon South Leitrim.

Voters across parts of the counties in the Midlands and north west of Ireland voted 51.42% against extending marriage rights to gay people.

The official figures showed a 61% turnout in the constituency with 18,644 people marking an X in the No box.