Cassin began her broadcasting career working for Dublin pirate radio station Radio Nova in 1982,  where she co-presented a nightly news programme, Dublin Today, with Bryan Dobson, later a fellow newscaster on RTÉ. She began presenting radio and television news bulletins for RTÉ in 1998 and has also covered current affairs, politics and sport as a reporter.
Cassin has also presented various non-news programmes including the weekly Dublin-based feature series Capital D for six years, Crimecall and The All Island School Choir Competition. On 20 December 2011, she was named as the new co-presenter of Nationwide, replacing Michael Ryan. Her first Nationwide broadcast occurred on Friday the 13th of January 2012.