In Irish O Caiside, "descendant of Caiside", from Cas, meaning "curly-headed", the surname is inextricably associated with Co. Fermanagh, where the family were famous for centuries as poets, churchmen, scholars and as hereditary physicians to the great Maguire chieftains. As these occupations attest, unlike many other prominent Irish families they were essentially peaceful; the exploits for which they are recorded in the Annals are healing, poetry and piety. In Fermanagh, their original seat was at Ballycassidy, north of Enniskillen. Two other placenames in the county also record their importance, Cassidy in Derryvullan civil parish, and Farrancassidy in Inishmacsaint. As their healing skills became widely known, many Cassidys were employed by other chiefs, particularly in the north of the country, and the name is now particularly common in counties Donegal, Monaghan and Antrim, as well as in the original homeland of Fermanagh. The final collapse of the old Gaelic order in the seventeenth century left them, like so many others, without their traditional role. Although less numerous elsewhere, the name is now also familiar throughout Ireland, with the smallest numbers to be found in Connacht.
In 1890 141 Cassidy births were recorded, the vast majority in counties Donegal, Antrim and Fermanagh, with a significant number also in Dublin.
Giolla Mochuda Mor O Caiside (d. 1143) was one of the earliest, and best known, poets of the family.
William Cassidy (1815-1873) was the grandson . of an Irish emigrant and rose to become an eminent American politician.
The mother of Bill Clinton, 42nd President of the U.S.A. was Virginia Cassidy, who married Roger Clinton when Bill was four years old.
.Neal Cassady (d.1967) was an friend of, and inspiration to, many of the American "Beatnik" writers of the 1950s and appears under various aliases in the novels of Jack Kerouac. He was the creator of one of the world?s most perfect puns, a road sign outside Ken Kesey?s house which read "No left turn unstoned."