Born Theresa Scott, in Golden, County Tipperary, she was educated at University College Dublin and Maynooth College, training as a Teacher. Ahearn lived with her husband Liam and family on the family farm near Clonmel.
Her first elected office was as a member of South Tipperary County Council from 1983 until 1999. Noted as a highly effective orator, she was elected to the 26th Dáil as a Fine Gael Teachta Dála (TD) for Tipperary South at the 1989 general election, becoming the only female Fine Gael deputy representing a rural constituency in the Dáil. She was re-elected at the 1992 and the 1997 general elections.
In Dáil Éireann, she was appointed as a Fine Gael spokesperson on Labour in 1992, and from 1992–1993 on Energy. She was the spokesperson on Women's Affairs and Chairperson of Oireachtas Committee on Women's Rights 1993–1995. She had been the Fine Gael Junior spokesperson on Equality and Disability since September 1997. Ahearn was a member of the Joint Committees on Foreign Affairs and on Justice, Equality and Women's Rights, and of the Committee on Procedure and Privileges.
Ahearn died of cancer on 20 September 2000 at Dublin's Mount Carmel Hospital, after a long illness, and was survived by her husband Liam and four sons. At the time of her death she was both a member of Fine Gael's National Executive Committee, and the first-ever female trustee of the party.
In the by-election after death, her Dáil seat in Tipperary South was retained for Fine Gael by Tom Hayes of Kilfeakle. She was the second TD in that constituency to die in 2000, the Labour Party's Michael Ferris having died on 20 March.
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- "Tricolour at half mast as parties pay their tributes". Irish Independent. 21 September 2000.
- "Theresa Ahearn". ElectionsIreland.org. Retrieved 15 January 2011.
- "Tributes paid to Fine Gael TD Theresa Ahearn". RTÉ News. 20 September 2000.
- "Theresa Ahearn TD dies after a long illness". ElectionsIreland.org. 20 September 2000. Archived from the original on 26 September 2007.