The Wicklow TD left the Social Democrats last year
The Fianna Fáil parliamentary party has unanimously accepted Stephen Donnelly's application to join.
The Wicklow TD was formally welcomed to the party at Leinster House Thursday afternoon.
He says his supporters in Wicklow wanted him to make a difference - and that is why he is joining Fianna Fáil.
Mr Donnelly was unveiled as the party's new Brexit spokesman.
In a press conference, he said he is more worried about the country now than he was when he first entered politics six years ago.
But he also said he will not retract his previous comments where he criticised the party in which he now serves.
While speaking to Newstalk Drive, Mr Donnolly said the party is 'a good fit'.
“Fianna Fáil is the obvious fit from a policy perspective.
“I see in them – I see in Michael and I see in the team – some new thinking.
“I see very clear views on Brexit, I saw a very clear stance on the Muslim ban that was taken during the week in the Dáil.
“These are really serious times, and so I saw a very good fit with Fianna Fáil – I’ve been impressed with Michael as a politician with his integrity and his policy stances.”
He also answered questions in relation to his previous anti-Fianna Fáil tweets on the party's record in government:
Is it just me, or is FF now talking about sharing power with FG? But they promised, unequivocally, that they wouldn't. They. Promised.— Stephen Donnelly (@DonnellyStephen) February 27, 2016
@iamDarragh Really? Take a look at Fianna Fail policies for the past 15 years and tell me you don't see serious incompetence.— Stephen Donnelly (@DonnellyStephen) November 14, 2011
In a post on his website back in 2014, he suggested the party was “jobs for the boys, bonuses for the boys, lack of accountability and two fingers to the Dáil”.
Deputy Donnelly was originally elected as an independent, before becoming one of the founders of the Social Democrats in 2015.
He was one of three Social Democrats elected to the Dáil last year, but in September Deputy Donnelly announced he was leaving the fledgling party.
In a statement earlier, Deputy Donnelly said: "I was motivated to enter politics to make a difference in my community and to try and help change the direction of the country.
"I am very concerned at the various challenges facing our country and believe I can best play my part in addressing them as part of the Fianna Fáil team whose policies reflect what I have advocated for."
He added that the party has "developed a suite of constructive, progressive policies that have emphasised fairness and investment in public services".
Fianna Fáil leader Micheál Martin said: "Stephen has made a serious impact nationally in his time as a TD. In his various roles he has demonstrated a commitment to the principles of fairness and social democracy that underpin the values and policies of the Fianna Fáil party and I know that he will be a strong addition to our existing team."
He will join Fianna Fáil's existing Wicklow TD, Pat Casey.
Social Democrats co-founder Catherine Murphy says she actually thought Deputy Donnelly would have joined another party sooner.
"I've got to say I'm not hugely surprised," she said.
"It's probably some surprise, if anything, that it didn't happen a little bit sooner. But that's how things are."