Ireland is to vote in 2015 on introducing gay marriage.
A referendum on the major social reform will come five years after same-sex couples were first given the right to c ivil partnership by the Government.
Kieran Rose, chairman of the Gay and Lesbian Equality Network (Glen), said the decision to hold a referendum was another historic step in 20 years' campaigning.
"The next step to ensuring equal citizenship for lesbian and gay people is to provide equal access to civil marriage through a Constitutional referendum," he said.
The decision on the referendum was revealed following a cabinet meeting of the Irish Government after it had been recommended by the country's Constitutional Convention, a 12-month long examination into reforming areas of Irish politics and society.
It also comes 20 years after homosexuality was decriminalised.
The convention recommended in July that the constitution be amended to allow for same-sex marriage.
More than 1,500 couples have secured civil partnerships in Ireland since they were introduced in 2010.
Noeleen Hartigan, programmes director for Amnesty International Ireland, sa id she hoped other human rights would also be looked at by the convention.
"This is the first step towards true marriage equality in Ireland. It is a tremendous achievement by the LGBTI campaigners and rights groups which have struggled so long to achieve recognition for this fundamental human right," she said.
Grainne Healy, chairman of Marriage Equality, said: "Marriage Equality is confident that the people of Ireland overwhelmingly support the extension of civil marriage rights to lesbian and gay people.
"This referendum is unlike most other referenda, it's not concerned with politics or economics, it's about Ireland valuing its citizens equally. Introducing marriage equality to Ireland would strengthen our reputation as champions for human rights and equality."
Mark Kelly, director of the Irish Council for Civil Liberties, said the support from the Constitutional Convention for the change in law was a clarion call for equality.
"When the people of Ireland vote on this issue in 2015, we will be participating in a final act of legal recognition of the full equality of our gay and lesbian colleagues and neighbours, friends and family," he said.
The Iona Institute, which describes itself as a pro-marriage, pro-religion think-tank, said bringing in the reform was about redefining marriage.
Spokesman Dr John Murray said: "This debate is really about the value we attach to a child having a mother and a father as distinct from two fathers or two mothers."
The Union of Students in Ireland said it would be asking its members to vote in favour in a referendum.
Aodhan O Riordain, Labour TD, said the decision was a hugely significant step.
"However this journey is not over, all of us on the yes side must now come together and work hard to ensure its passage in 2015," he said.
"Those opposed to equality will do their best to confuse and distort the arguments so it is essential that we remain true to the central argument in this campaign, which is that gay and lesbian people should be entitled to offer their partners the love and protection that they need and deserve."
The referendum will take place in the middle of 2015.
Proposed legislative changes dealing with same-sex marriage will be published by the Government before Christmas with Justice Minister Alan Shatter examining issues around marriage and Children's Minister Frances Fitzgerald dealing with issues on adoption by gay couples.
"This is a matter that the Government considered this morning and accepted the recommendation of the Constitutional Convention that the question of gay marriage should be put to the people," Taoiseach Enda Kenny said.
"What Government decided was that by mid-2015 this question and a number of others would be put to the people."