Eamon Ryan's use of a racial slur has intensified the internal split in the Green Party that is destabilising efforts to form a government.
As talks between Fianna Fail, Fine Gael and the Greens to finalise a programme for government continued late last night, Mr Ryan was facing calls to drop out of next month's leadership contest after he used the 'N-word' during a debate on racism in Irish society.
Mr Ryan said in the Dail: "In the newspaper today there was a young Irish man Sean Gallen giving his experience of being 'othered', from the age of six being given that name, 'You n*****'.
He later apologised on Twitter: "In quoting from an article I read this morning, I repeated a racial slur, and I was completely wrong to do so. I want to apologise for any hurt caused. I know this particular word should never be used."
Mr Ryan's latest gaffe sparked turmoil in the party, with Green Party councillors publicly condemning him and supporters of his opponent, deputy leader Catherine Martin, saying it underlined the need for a change of leader. Councillor Daniel Whooley told the Herald that Mr Ryan should drop out of the leadership race.
However, Fianna Fail and Fine Gael moved to quell the controversy.
Justice Minister Charlie Flanagan said: "There is not a milligram of racism or prejudice in Eamon Ryan's blood. We need an informed sense of perspective here."
A number of Fianna Fail politicians also tweeted their support for the party leader.
The furore comes at a time when Green Party councillors are overwhelmingly open to going into government with Fianna Fail and Fine Gael, according to a Herald survey.
Almost 70pc of the party's 49 councillors have said they are not opposed to coalition with the civil war parties in principle, though the majority said it would depend on the deal.
Many elements of that deal were still not agreed by the parties last night as Mr Ryan attempted to quell internal disquiet by contacting senior party figures.
A senior Green Party source said Mr Ryan had now committed a string of errors that had made his position more difficult. "It isn't just this gaffe, it's the cumulative factor of the M20, lettuce, wolves, car-pooling etcetera," the source said.
Mr Whooley said Mr Ryan's comments were "morally reprehensible", and added: "He should consider dropping out of the leadership race with comments like this. A lot of people are disgusted."
A number of councillors condemned the remarks. However, others backing Mr Ryan include former party chair Dan Boyle, who said anyone who knows him would never "attribute any malice" to the comments.