Update at 10.30pm: Stephen Donnelly has denied his opinion on the Apple tax appeal was the reason for leaving the Social Democrats.
The TD, who only co-founded the party 14 months ago, has today resigned from the party.
However the Wicklow TD had voiced some disagreement with the party's policy, which opposes an appeal of the €13 billion ruling.
He said this evening his position was slightly more nuanced than that of the party: “I had a slightly different view, fundamentally very similar, Apple should have been paying their taxes, the Irish State should not have been facilitating this massive tax avoidance, I just would be concerned that this allows the commission to start meddling in our corporation tax affairs.
Deputy Róisín Shortall denies that the party will now face a struggle to survive: “The fact of the matter is that the Social Democrats were in existence for a mere six months, we set up without any state funding at all, without any backing.
“We ran a very credible campaign we believe, we influenced the direction of the debate during the campaign and Steven played an important role in that."
Update 5pm: Stephen Donnelly's office says he will stay as part of the Dáil group alongside his former Social Democrats colleagues and the Green Party, in order to maintain speaking rights.
He said the decision to leave the party was "a very difficult personal decision".
His former party colleague Catherine Murphy said continuing the work as a technical group in the Dáil was the right thing to do and added: "We've all go to be mature about this."
Update 3.20pm: Social Democrats co-founder Catherine Murphy has said she is “not surprised” that Stephen Donnelly has left the party.
Deputy Murphy said the Wicklow TD had been "disengaged" for some time, and was taking August to consider his future.
“It hasn't come as a complete bolt from the blue…He was taking August off to think about things,” she said.
“(However), we’re determined to plough ahead with the development of the party.”
Fellow co-founder Róisín Shorthall said: "We're really building the capacity of the party and we believe it has a very bright future. It's a pity he's not a part of that, but we have to respect his decision and we wish him well"
Update: 1.15pm: Stephen Donnelly has citing leadership issues as the reason for his leaving the party.
The Wicklow TD said that his partnership with Roisín Shortall and Catherine Murphy as leaders of the party did not work, saying: "Some partnerships, in every walk of life, simply don’t work."
He said that: "In spite of everyone’s best efforts, I have concluded that our partnership did not have that."
He statement said: "I am announcing, after a prolonged period of consideration, that I am leaving the Social Democrats with immediate effect. I do so 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.
"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.
"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.
"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. To this end, I will now be consulting with my supporters in Wicklow and East Carlow on the best way forward..
"I wish to thank most sincerely the many, many people who support both the Social Democrats and me personally.
"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."
Earlier: A founding leader of the Social Democrats has left the party.
Stephen Donnelly - who is a TD for Wicklow - was leader alongside Roisín Shortall and Catherine Murphy since it was founded in July of last year.
All three were the only TDs returned for the party at the last election.
A statement from the Social Democrats says they are surprised that he has decided to walk away from the project, but the remaining members will continue to build the party.
Statement from Social Democrats regarding the departure of Stephen Donnelly T.D. https://t.co/fW6R6cQ0OF— Social Democrats (@SocDems) September 5, 2016
The full statement by the Social Democrats read:
"Stephen Donnelly T.D. has informed us that he is leaving the Social Democrats.
"We are disappointed that he has decided to walk away from the project, we undertook, to establish and build the party.
"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.
"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.
"We wish Stephen the best in his future endeavours and look forward, with excitement, to the future development of the Social Democrats."