Mr Donnelly, who was a founding member of the Social Democrats was criticised by his former party colleagues after stepping down yesterday.
Both Róisín Shortall and Catherine Murphy accused the Wicklow TD of a lack of commitment to the party after he announced his resignation and claimed he had not put in the hard work required when leader of a party.
But the Harvard Kennedy School graduate hit back at his former party co-founders and said: “I think it is an unfortunate line that Catherine and Róisín are taking, nobody that starts a new political party is work shy.
“It just wasn’t working well enough for me.
“I certainly have been accused of a whole manner of things in my time but no one has ever accused me of being work shy.
“We all worked very, very hard on this,” he said.
After announcing he was leaving the party which was established in July of last year, Ms Murphy said Mr Donnelly would admit himself that he had been “disengaged for some time” and had not put in the “non-glamorous” work required to get a new political group off the ground.
The Social Democrats won three seats in this year’s general election and gained a 3% share of first preference votes but Ms Murphy said Mr Donnelly had not been committed to getting the party fully established.
The Kildare North TD said: “He had been disengaged for some time so we were aware that he wasn’t happy or that it wasn’t working for him.
“We have been ploughing ahead with some of the non-glamorous work in the background doing the kind of things that are the foundations for a party, like writing the draft constitution and getting the membership up and running.
“You need a foundation if you are going to build a house, if you don’t get the foundation right then the rest of it won’t work out and some of that is just drudge and essentially that is where we have focused our attention in recent months.”
Fellow founding member and former junior health minister Ms Shortall agreed that Mr Donnelly may have had “different expectations” as to the amount of commitment and work needed to fully establish the party.
But she said the leadership issue, which the party initially said it would review after the election, “never arose”. “We decided on having a three-person leadership from the very beginning because of the amount of work involved in it.”
Ms Shortall said: “Maybe he had different expectations as to what it would be like to start a new party, to build a party from scratch, it’s tough work building it block by block, it requires very long hours and a very strong commitment by a large number of people.
“Sometimes you have to be patient in doing that,” she told RTÉ.
In a statement announcing his resignation from the party Mr Donnelly said: “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.”