The 21-year-old Dublin-born Cassidy started out with North Dublin-based St Kevin's Boys.
Championship side Watford spotted him and he moved to Vicarage Road in July 2017, signing his first professional deal in 2018.
After signing a new two-year contract with Watford in July 2020, he moved on loan to League One side Accrington Stanley in August 2020.
He totalled 17 games for Stanley and scored twice in an EFL Trophy tie against Leeds United under-21s in September 2020.
In January 2022, he returned to Ireland to join League of Ireland Premier Division side Bohemians, for whom he played 8 times.
He was released by Watford at the end of the 2021/22 season at the expiry of his contract.
Cassidy has represented Republic of Ireland at under-17, 18 and under-19 level.
He received his first official call up to the under-21 squad in November 2020.
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