Socialist TD Joe Higgins will not contest the next general election.
He was reportedly thinking of standing down at the end of this Dáil term, however the forthcoming Dublin West by-election has brought the decision forward.
His party's candidate is a favourite to win the seat and Deputy Higgins is questioning whether the party could take two seats in the following general election.
He has told the Irish Times it is better to clarify the position now.
He said: "When the next general election arrives in 2016 I will be 67, and I am very anxious that a new generation of Socialist Party public representatives are continuing, if you like, the work that the party and I have done for quite a lengthy period."
Mr Higgins said he would still be active in politics.