Cassidy was born in Castlepollard in County Westmeath. He came to prominence in Ireland through the showband scene. A saxophone player with Jim Tobin and the Firehouse, he moved into showbusiness management. He was the manager of Foster and Allen, a popular singing duo that enjoyed success inside and outside Ireland.
Cassidy owns two Dublin hotels and has another on the former site of the National Wax Museum, which he also owned. He also owns Celtic Note, a specialist Irish music store on Dublin's Nassau Street and since 1984 has had the franchise to sell CDs and DVDs at Dublin Airport. He also owns several buildings in Mullingar. His other interests include record and music publishing companies. He owns the publishing rights to many well known songs such as 'Grace' and 'My Lovely Rose of Clare'.
In a debate in Seanad Éireann on 10 April 2008, Cassidy stated the following about house prices in Ireland: "We have a duty to tell first-time house buyers, young couples with no previous experience, that there is unbelievable value in the marketplace today. It will not last forever. It is never the wrong time to do the right thing. I offer the House the benefit of my experience and my opinion which is all any Member can do. I will remind the House, perhaps in 12 or 18 months, when prices have again increased by 25% or 30%, that they were told this by the Leader of the House on this historic day, the tenth anniversary of the Good Friday Agreement." In the event, 12 months later the average house price had decreased by 21.8%.