Cassidy began his career as a journalist, editing a community newspaper in Dublin North West. He moved to Australia and started a newspaper for the Irish community there. A small operation, Cassidy was involved in the writing, photography and graphic design of the publication. Back in Ireland he was managing director of a multimedia company that had a training wing with a portfolio of courses including tv and film production. Cassidy came to realise that film brought together his hobby of music engineering with the skills he had been honing in photography and writing. His first endeavours in film were in the area of documentary. One of his first films was "War on waste", a documentary on recycling shot in Mumbai, India. It was when a script he wrote was commissioned by Amnesty International that he first worked in drama. He was then successful in securing a commission for his first Irish Film Board funded short "Whatever Turns You On" that was a notable success, scooping over 10 international awards.
Cassidy has made a number of short films, docmentaries as well as an eight-part television drama series "The House" which was funded by the Broadcasting Commission of Ireland. This four hour work of drama saw Cassidy direct a cast of 67 actors.
Cassidy has also created a digital film curriculum for a transnational European conglomerate that has been delivered to students in six countries. His current project is a novel, Pins and Needles, which will form the basis for a feature film by the same name.