Cllr Deirdre Heney will contest the Dublin Bay North constituency for Fianna Fáil after beating Cllr Sean Haughey at the party’s selection convention tonight.
148 delegates voted and it is understood there was only a small number of votes between them, with unconfirmed reports suggesting only two votes separated the candidates.
Mr Haughey had been widely tipped to win the convention.
The party had agreed to select one candidate and it was understood they would add another at a later stage.
However, it is not clear this evening if a second candidate will be added to the ticket.
Earlier, two other candidates - Tom Brabazon and Aileen Woods - withdrew from the convention.
Last month Senator Averil Power resigned from the party. Fianna Fáil leader Micheal Martin linked it to the contest for the convention.
However, Senator Power cited the party's position on the same-sex marriage referendum as her reason for leaving.