The Irish surname O'Cassidy is of patronymic origin. The name is the anglicized form of Gaelic name Ó Caiside meaning “descendant of Caiside” which is a byname from cas meaning “curly(-headed).” The O'Cassidys were specifically from County Fermanagh in the province of Ulster and they provided physicians to the Kings of Ulster, the Clan Maguires particularly in the medieval period between the years 1300 and 1600. In fact at this time and through to the 19th century the O'Cassidy's were renowned for their education, providing a high proportion of the members of the priesthood in the region. The O'Cassidys also excelled in the field of literature, Rory O'Cassidy, Archdeacon of Clogher, is said to have helped compile the 15th century "Annals of Ulster", whilst Thomas Cassidy, an expelled friar of the Augustine order in the 18th century was not only a soldier of fortune in the Army of France, but wrote a number of books about his experiences.
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