Legislation to give full legal recognition to same-sex couples is to be reintroduced in the Dáil this week by the Labour Party in a Private Members' motion.
The move challenges and could embarrass the Green Party, which supported it in Opposition when it was first introduced by Labour's Brendan Howlin.
However, Green Party justice spokesman Ciarán Cuffe said he would have discussions tomorrow with Minister for Justice Brian Lenihan about the issue.
"We're in favour of full equality for those in same-sex relationships," he said yesterday.
"There is a debate about the best legislation. We want legislation along the lines of the Colley report, the essence of which is equality."
There have been contacts between the Minister and the Green Party in the past several weeks and Mr Cuffe said that, while there could be legal complexities over matters such as succession rights, the issue "boils down to the Programme for Government, which states that legislation shall be introduced".
However, Mr Howlin, now the party's spokesman on constitutional matters and law reform,said he was tabling the motion to reactivate the Bill because, "while the Programme for Government says that the coalition will legislate for civil partnership, there is no sign of it attaching any priority to this issue at all".
When he introduced the Bill in the Dáil in February it was supported by the Green Party, "and was praised by many individual TDs on the Government benches", Mr Howlin said.
He added that then-minister Michael McDowell "used a procedural device to have the second reading postponed for a period of six months. He was well aware that this would be after the general election and that the Labour Bill would fall with the dissolution of the 29th Dáil."
The Labour Bill provides that the laws applying to marriage would also apply to civil unions.
The Constitution states that the State "pledges itself to guard with special care the institution of marriage on which the family is founded and to protect it against attack". The Labour Party believes the Bill is in compliance with the Constitution.
During the February Dáil debate, Mr McDowell said the Labour Bill would not comply with the Constitution but the Government would introduce legislation later in the year.
In July, Taoiseach Bertie Ahern said he hoped to move "as quickly as possible" to bring in new legislation to legalise civil partnerships.
Mr Howlin believed "that there has now been sufficient time devoted to consultations and discussion of the issue. Gay and lesbian citizens are entitled to expect the Dáil to legislate to ensure that have the same basic rights as other citizens."
The Wexford TD said he was "confident that Irish society has matured and attitudes changed sufficiently to the stage where a majority of our people would have no problem with legislation that would provide same-sex couples with the same rights and duties that are generally applied to married couples".
