Growing up in Dublin, Bhreatnach joined Sinn Féin at the age of sixteen and was subsequently active in the Anti H-Block campaign, becoming the Chair of the Stop Strip-Searching Campaign. She is the daughter of the journalist the late Deasún Breathnach. She worked for the Sinn Féin newspaper An Phoblacht in 1982, then on the Australia Desk in the party's Foreign Affairs Bureau.
In 1986, Bhreatnach was elected to the party's Ard Chomhairle, and in 1988, she became its General Secretary. At the 1994 European Parliament election, Bhreatnach stood in Leinster, but took only 2.5% of the votes cast and lost her deposit. At the Northern Ireland Forum election in 1996, Bhreatnach did not stand in a constituency, but was elected as she was placed first on Sinn Féin's "top-up" list.
Bhreatnach was part of the Sinn Féin delegation to talks at Downing Street in 1998 in the lead-up to the Belfast Agreement. From 1999 until 2002, she headed the party's Electoral Department, opposing the Nice referendum and planning for the 2001 UK general election. She stood down as General Secretary in early 2003, moving to a new role, reorganising the party's Roinn an Chultúir and worked as head of its Equality Section, focussing on gender equality. In 2003, she organised a Sinn Féin conference, "Engine for Change — Women and Equality"
As of 2005[update], Bhreatnach worked as a journalist. In December 2007, she was appointed to the all-Ireland Irish language body Foras na Gaeilge, and represents that body on the Audience Committee of RTÉ.
- "Céad Bliain - Breithlá Shinn Féin: Celebration dinner 5 November", An Phoblacht, 27 October 2005
- ElectionsIreland.org: 1994 European - Leinster First Preference Votes
- The 1996 Forum Elections: Regional List of Candidates, Northern Ireland Elections
- "We will stay in talks - Sinn Fein", BBC News, 19 January 1998
- "Bhreatnach takes on cultural brief", An Phoblacht, 25 July 2002
- Roisin de Rosa, "Women in history- Engine of Change", An Phoblacht, 6 November 2003
- "Boards of North South bodies appointed today", Northern Ireland Executive, 17 December 2007
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