The daughter of former Fine Gael Teachta Dála (TD) Michael Joe Cosgrave, she was briefly a member of the Seanad Éireann. After standing unsuccessfully in the Dublin North Central at the 1997 general election, she was one of eight new senators nominated by outgoing 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 Corporation as a councillor for the Donaghmede ward. She was re-elected in the 2004 local elections to the renamed Dublin City Council, but in July 2005, the Fine Gael party withdrew the party whip from her after she did not vote 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 removed from membership of the council for non-attendance, having attended no meetings for six months. She was replaced by Pat Crimmins.