Stephen Donnelly, one of the founders and co-leaders of the Social Democrats, has left the party in bitter circumstances just over a year after the party was established.

Mr Donnelly, a Wicklow deputy, is a prominent media performer and was one of only three TDs for the party, along with Roisín Shortall and Catherine Murphy, who were also his co-leaders.

A spokeswoman for the party said it had not come as a surprise and Mr Donnelly had been disengaged for some time.

In a statement the party said it was “disappointed that he has decided to walk away from the project, we undertook, to establish and build the party”.

In an apparent swipe at Mr Donnelly, it also said: “The levels of dedication required for such a major undertaking can be overwhelming for some.

“However, our elected councillors, our staff team and our volunteers are passionate about our project and we will now get on with the job of building our party.”

Prior to the foundation of the Social Democrats, Mr Donnelly had been associated with a move to Fianna Fáil, which he denied at the time.

Ms Shortall said Mr Donnelly said he intended to become an Independent TD. Certain Fianna Fáil sources said that if “someone wants to join, they apply but we are not scouting”.

Mr Donnelly did not return calls today, but released a statement saying he is leaving “after a prolonged period of consideration” and also countered that the leadership team was not working.

“It is a fact that some partnerships, in every walk of life, simply don’t work no matter how hard all of the parties to that partnership try to make it succeed.”

He said the partnership with Ms Shortall and Ms Murphy did not function very well. “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.

“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 said he is leaving “with great sadness, having vested so much together with my parliamentary colleagues, Catherine and Roisin, a small core team and many volunteers across the country, into the establishment of the Social Democrats over the last 20 months”.

Mr Donnelly said he will be “consulting with my supporters in Wicklow and East Carlow on the best way forward” in his political career.

"I entered politics in 2011 because I saw an urgent need for new thinking and approaches to steering Ireland through turbulent times and into a new era of opportunity and equality. I remain just as committed to making as best a contribution as I can to my constituency and our country through politics, as a social democrat.

“I will continue to work hard on the issues I am championing, including stopping tax avoidance by vulture funds, improving public services, resolving the mortgage crisis; responsible fiscal management, promoting high quality job creation and reducing the costs of living and doing business.”

The Social Democrats statement added: “The Executive Committee of the Party has reaffirmed its commitment to the vision of a strong economy, fair society and honest politics.

“We are fully committed to the project and will endeavour to develop the Party into an exciting force for change in Irish politics.

“As is the case across the globe the defence of social democratic values is not dependent on one personality or politician - but rather is a collective pursuit.

“Since the Social Democrats were first founded, in July 2015, we have worked to build the party brick-by-brick. 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.

“We wish Stephen the best in his future endeavours and look forward, with excitement, to the future development of the Social Democrats.”