Cassidy was elected to Dáil Éireann for the Westmeath constituency at the 2002 general election, taking the seat of sitting TD and Government Minister Mary O'Rourke. This created a rift between the two politicians and Cassidy lost his seat to O'Rourke at the 2007 general election when both candidates contested the Longford–Westmeath constituency. The fact that Cassidy lost most of his home base of Castlepollard and north Westmeath due to the redrawing of the constituency did not help him. On 22 June 2007, Cassidy was nominated by the Taoiseach Bertie Ahern to Seanad Éireann and was appointed Leader of the Seanad for the remaining weeks of the 22nd Seanad's existence. Cassidy was subsequently elected to the Labour Panel at the 2007 Seanad general election and was Leader of the Seanad until March 2011. He lost his seat at the 2011 Seanad election.
