[2] He was elected to Seanad Éireann in 1989 as a Senator for the Industrial and Commercial Panel, serving as Cathaoirleach, and was re-elected in 1993 and 1997.
On 17 October 2005, Cosgrave pleaded guilty to a charge of failing to disclose to the Standards in Public Office Commission a political donation of £2,500 received from lobbyist Frank Dunlop in the course of the 1997 general election.
[5] Cosgrave appeared at the Mahon Tribunal on 14 July 2006 in relation to a planning application from Monarch Properties to re-zone 236 acres (0.96 km2) at Cherrywood, Dublin.
An investigation of corruption was being instituted on the basis of bribes allegedly paid to councillors by Frank Dunlop to secure re-zoning on 16 December 1997.
[8] He went on trial in the Dublin Circuit Criminal Court on 4 July 2013, on five counts of corruptly receiving money on dates between June 1992 and December 1997.