Former Fianna Fail TD Chris Andrews has said his decision to join Sinn Fein was based on the party's commitment to pursue the changes that Ireland needs.
The national executive earlier accepted his application to join its ranks just a year after he left the party his grandfather founded.
"But it has become very clear to me that the party no longer represented the ideals or values of its founders, including my grandfather," he said.
"I left Fianna Fail in 2012 following a long period of disillusionment at how distant the party had become from ordinary people suffering the brunt of social and economic problems.
"I have joined Sinn Fein because I firmly believe that it is genuinely committed to pursuing the political, social and economic change that Ireland now requires.
"My decision has been influenced by my first-hand experience of Sinn Fein's work at community level, its role in the Peace Process, its strong commitment to a united Ireland and its track record on international solidarity.
"As someone who is advocating change I must be willing to change personally and politically."
Mr Andrews, who served as a TD for Dublin South-East, is hoping to run in the next local elections in the Ringsend area.
He said the problems that Ireland faces require a major political and economic change.
"I do not believe that such change can be delivered through Fianna Fail, Fine Gael or Labour," he added.
"Nor can it be delivered by 'Independents'."
Mr Andrews is the nephew of David Andrews, a former Fianna Fail minister who helped negotiate the Good Friday Agreement, and his grandfather Todd Andrews helped found Fianna Fail.
He resigned from Fianna Fail last year following a social media controversy and clash with party leader Micheal Martin.
It emerged the 49-year-old set up a phoney Twitter account which he used to criticise Mr Martin and other figures within the party.
He was discovered after Eddy Carroll, husband of senior party figure Kathryn Byrne, used video and photo surveillance to catch him in the act.
Sinn Fein party whip Aengus O Snodaigh said he was pleased to welcome Mr Andrews into the party.
"Both Chris and I have worked on campaigns together and indeed were shipmates on the 2008 and 2011 freedom flotillas to Gaza," he added.
"I have known Chris for a long time and regard him a person genuinely committed to building a better country for all our citizens."