The Progressive Democrats have chosen a 27-year-old residential care worker Ms Sirena Campbell to contest the forthcoming by-election in Meath.
Ms Campbell, from Julianstown, Co Meath, was selected unopposed at a convention in Navan last night.
A residential care worker at Trinity House detention centre, she is studying for a masters degree in equality studies at UCD.
She was an unsuccessful local election candidate last June, but secured close to half a quota in the Julianstown area.
The by-election has been necessitated by the appointment of Mr John Bruton as the EU's representative in Washington.
Other candidates will include Cllr Tommy Reilly (FF), Mr Shane McEntee (FG), Mr Dominic Hannigan (Lab), Mr Joe Reilly (SF) and Mr Fergal O'Byrne (Green Party).
The by-election is expected to take place during the spring, on the same day as one in Kildare North caused by the appointment of Mr Charlie McCreevy as European Commissioner.
Speaking at the convention last night, the Tánaiste and PD leader, Ms Harney, said Ms Campbell was "an excellent candidate for the Progressive Democrats and is exactly the kind of person needed to give Meath a strong voice.
"She has greatly impressed everyone in the party with her energy, enthusiasm, her drive and commitment. We need more dynamic young people like Sirena in politics, determined to make a real difference."
Ms Campbell said she was "honoured to have been chosen to run for the Progressive Democrats. I believe that I have the energy and the talent to provide Meath with strong and effective representation in Dáil Éireann.
She said politics was about making tangible and positive differences to the lives of ordinary people. "That's what I have tried to do with my work in the community and what I believe I can do more effectively as an elected politician."
The Minister of State for Finance, Mr Tom Parlon, who chaired the convention last night, is to be the party's director of elections for the Meath constituency.