Cassidy (Irish: Ó Caiside / Ó Casaide) is a common Irish surname and is sometimes used as a given name.
The surname translates to "descendant of Caiside".
Variations include: Cassady, Cassiday, Cassedy, Casadei and Cassedey.
The family was originally a Munster sept called Uí Chaisín[1] but in the 12th century a branch moved to Devenish Island in County Fermanagh,[1] where they became a medical and poetic family, hereditary physicians to the Maguires.