Councillor Patrick McKee has said he decided to join Renua after deciding to withdraw his name from Fianna Fáil's selection convention to choose a candidate to contest the upcoming Carlow-Kilkenny by-election.
He said he had become "disillusioned" with party politics.
The 26-year-old from Kilkenny city will contest the Carlow-Kilkenny by-election on 22 May as a Renua Ireland candidate.
The ballot is to fill the Dáil seat vacated when Phil Hogan became European Commissioner for Agriculture.
Speaking on RTÉ’s Today with Sean O'Rourke, Mr McKee said he was disappointed that Fianna Fáil had not taken the opportunity to implement significant reforms since the economic crash.
Fianna Fáil finance spokesperson Michael McGrath said Mr McKee may have seen a greater opportunity within a smaller party.
He said Cllr McKee's decision was a personal one and he was disappointed but he wished him well.
He said the leadership of Fianna Fáil was not an issue, adding that nobody was holding a knife over anyone threatening them if the result of the by-election goes a certain way.
Mr McGrath said he had confidence in Michael Martin's leadership and that the party has got all of the big issues right, including the economy.
He told RTÉ's News At One that there is a collective determination to build on the renewal of the party, but that recent opinion polls have certainly been disappointing, adding that there is no point in shying away from that.
He said when it comes to a general election voters will look at a mainstream party that he said has called all the big decisions right.
Laois-Offaly TD Sean Fleming said Fianna Fáil wanted to win the by-election and two seats in the general election in Carlow-Kilkenny.
