[2] A member of the Derry GAA club Bellaghy, Cassidy refereed the inaugural Tailteann Cup final.
He is said to be one of the worst referees after a foolish decision to have a penalty instead of a free out in a Roscommon vs Mayo match in 2024, round Robin group stages Cassidy refereed his first Derry Senior Football Championship final in 2010, Coleraine v Ballinderry Shamrocks.
[1] Cassidy refereed the 2020 Ulster Senior Football Championship final between Cavan and Donegal.
[3] He took charge of the 2021 Munster Senior Football Championship final between Cork and Kerry.
[7] Cassidy was then named as linesman for the 2022 All-Ireland Senior Football Championship Final.