The daughter of former Fine Gael Teachta Dála (TD) Michael Joe Cosgrave, she was briefly a member of Seanad Éireann. After standing unsuccessfully in the Dublin North-Central constituency at the 1997 general election, she was one of eight new senators nominated by the Taoiseach, John Bruton, to the 20th Seanad on 13 June 1997 to replace senators who had been elected to the 28th Dáil. After her nomination, the Seanad met only once (on 10 July 1997) before it was dissolved. In the 1997 Seanad elections, Cosgrave stood for election on the Labour Panel, but failed to win a seat.
At the 1999 local elections, Cosgrave was elected to Dublin City Council as a councillor for the Donaghmede ward. She was re-elected in the 2004 local elections, but in July 2005, the Fine Gael party withdrew the party whip from her after she abstained when Dublin councillors elected the next Lord Mayor of Dublin. In 2006, sitting as an independent councillor, she voted against Paddy Bourke, the Fine Gael-supported Labour Party candidate for Lord Mayor. This produced a tied vote tied between Bourke and the independent councillor Vincent Jackson, which was resolved in Jackson's favour by drawing names from a hat.
In June 2006, it was revealed that Cosgrave had the worst attendance record of any city councillor, having 17 out of 33 meetings in the preceding two years. In September 2007, she was deemed to have resigned her membership of the council for non-attendance, having attended no meetings for six months. She was reported to have moved to France. She was replaced by Pat Crimmins.
Cosgrave's political career continued until after she witnessed an RTÉ news report about contaminated blood, relating to anti-D. The report concerned women that had received the product in 1977. Recognising the symptoms after receiving anti-D during the birth of her second child, Cosgrave immediately presented for testing. She tested positive for hepatitis C and immediately began a campaign to have women that received the product in 1991 recognised. This was finally accepted by the Blood Transfusion Service Board and women infected in 1991 were acknowledged as recipients and received treatment and compensation.
Cosgrave was the first woman to go public about the failings of the BTSB and allow herself to be photographed by The Irish Times and contributed to the writing of a book Hep C Niamh’s Story by Fergal Bowers. Cosgrave gave interviews and appeared on The Late Late Show in support of her representation of victims. Cosgrave was also one of the first victims to undergo experimental treatment with interferon and, despite advice to the contrary, went on to have two more, healthy, children.
- "Niamh Cosgrave". Oireachtas Members Database. Retrieved 13 January 2008.
- "Mayor, councillor lose Fine Gael party whip". The Irish Times. 15 July 2005. Retrieved 14 January 2008.
- "Niamh Cosgrave". ElectionsIreland.org. Retrieved 13 January 2008.
- "Seanad Éireann debates, Volume 151, 10 July 1997: Nomination of Members". Oireachtas. Archived from the original on 7 June 2011. Retrieved 13 January 2008.
- Louise Healy and Tom Gilmore (27 June 2006). "Name of capital's new mayor pulled from a hat after tied vote". The Irish Independent. Retrieved 13 January 2008.
- "FF council members are worst attenders". The Irish Independent. 2 June 2006. Retrieved 13 January 2008.
- "Council expels absent Cosgrave". The Irish Times. 28 September 2007. Retrieved 13 January 2008.
- "Monthly Meeting of Council, 05/11/2007" (PDF). Dublin City Council. 5 November 2007. Retrieved 13 January 2008.
- Hep C Niamh's Story. Marino Books. ISBN 1860230539.