The Social Democrats have announced that its co-leader Stephen Donnelly has left the party.
The Wicklow TD was a founding member of the party, which was launched in July 2015 ahead of this year's general election.
The Social Democrats' three TDs and co-leaders – Stephen Donnelly, Catherine Murphy and Róisín Shortall – were re-elected, although none of their other candidates were elected to the Dáil.
In a post on his website, Deputy Donnelly said he was leaving the party with "great sadness" after a "prolonged period of consideration".
"My decision is based on the fact that for the Social Democrats to achieve its potential as a party of significant influence and scale, despite the many obstacles new parties face, one critical component is that the leadership team must function very well together as a team," he explained.
"In spite of everyone’s best efforts, I have concluded that our partnership did not have that. I further believe that this would be the case whether the leadership had continued to be shared or was vested in one person, which was not something I or anyone else had sought."
He thanked the "many, many people who support both the Social Democrats and me personally" and added he will consult with his constituency supporters on "the best way forward".
Deputy Donnelly will remain in the same Dáil group as the party, thus retaining its speaking rights.
In a statement, the Social Democrats said they were disappointed that their colleague had "decided to walk away from the project".
"We wish Stephen the best in his future endeavours and look forward, with excitement, to the future development of the Social Democrats," they said.
The party added: "This is a long-term project which requires dedication, hard work, long hours and a major commitment from all involved including our elected representatives.
"The levels of dedication required for such a major undertaking can be overwhelming for some."
Deputy Donnelly later disputed suggestions in the statement, telling Newstalk's The Right Show: "I thing it was an unfortunate thing to be said.
"Suffice to say, lazy people don't set up parties or run for election. I have fairly been accused of lot of things. I've never been accused of being work-shy. It's regrettable that they went there."
Social Democrats TD Róisín Shortall told Newstalk Lunchtime that his departure did not come as a shock.
"We knew Stephen had been considering his options for some time - probably since the election or formation of Government," she said.
"We've had some communication with him but not a huge amount over the past month or so. We spoke to him on Friday and he told us he had made his mind up at that stage."
She added: "I'm not sure that there's significant policy differences. I think it's more a case of it just wasn't for him, and maybe he has different ambitions in terms of politics."