Ireland's minority government looked set to collapse on Friday after the party propping it up submitted a motion of no confidence in the deputy prime minister, weeks before a summit on Britain's plans to leave the European Union.
The crisis is likely to lead to an election next month or in January and may complicate the December 14-15 Brexit summit.
Prime Minister Leo Varadkar is due to play a major role in the talks, telling EU leaders whether Ireland believes sufficient progress has been made on the future border between EU-member Ireland and Britain's province of Northern Ireland.
The border is one of three issues Brussels wants broadly resolved before it decides whether to move the talks on to a second phase about trade, as Britain wants.
Varadkar is now likely to go into the summit as a lame duck prime minister or in the middle of an election campaign.
The head of opposition party Fianna Fail, Micheal Martin, said his party had submitted a motion of no-confidence in Deputy Prime Minister Frances Fitzgerald, to be debated on Tuesday. The trigger is her handling of a legal case involving a police whistleblower.
Martin told state broadcaster RTE that an election "can be avoided if the government takes action" by asking Fitzgerald to resign. But the government said this would not happen.
Varadkar and Martin met briefly to discuss the standoff on Friday and are due to speak again on Saturday.
"At a time when issues and decisions will need to be made that will reverberate in our country for decades to come, the prospect of either an election taking place or a government not being in place afterwards is actually unconscionable," Finance Minister Paschal Donohoe told RTE.
"These are historic decisions ... The agenda that is at play here is one of political interest. We need to be aware of the consequences of the approach that Fianna Fail are taking here," the usually calm Donohoe fumed.
Junior Finance Minister Michael D'Arcy told Reuters he believed there would be an election before Christmas and a source familiar with the party's planning said it had begun to make preparations on Friday for an election.
As well as the border, the other issues Brussels wants resolved before talks move on to trade arrangements are Britain's financial settlement on leaving the bloc and the rights of EU citizens living in Britain.
EU Brexit negotiator Michel Barnier assured Irish Foreign Minister Simon Coveney on Friday that the EU would defend Dublin's position in talks with Britain over the coming weeks.
"Strong solidarity with Ireland," Barnier tweeted. "Irish issues are EU issues."
A snap election next month in the Irish Republic would also rule out a resumption of talks to restore a devolved government in Northern Ireland, the leader of the Democratic Unionist Party (DUP), Arlene Foster, told BBC.
The DUP has for months been in on-off talks with the Irish nationalists Sinn Fein to break a political stalemate north of the border. As Sinn Fein also fields candidates in the south, Foster said their focus would be entirely on that.
The Fianna Fail move comes after Fitzgerald admitted she was made aware of an attempt to discredit a police whistleblower in a 2015 email, but failed to act. Fine Gael say she adhered to due process.
The case relates to a whistleblower who alleged widespread misconduct in the force. His treatment by the authorities led in 2014 to the resignations of the then police commissioner and justice minister.