This famous Irish surname is an anglicized spelling of the original Gaelic O'Caiside - itself a compound with the elements "O" translating as "male descendant" and the personal name Caiside (derived from "cas" (curly hair), a nickname. 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. He was also known as MacCassidy, however this may well have been a nickname or even a disguise. The Cassidy's have long been opponents of British influence and rule in Ireland, William Cassidy who died in 1870 was an American politician who founded the United Irishmen of America, an organisation which used all its influence to prevent the USA assisting Great Britain in the World Wars. The first recorded spelling of the family name is shown to be that of Giolla Moduda O'Cassidy (Gaelic poet), which was dated 1143, during the reign of King Stephen, known as "The Count de Blois" 1135 - 1154. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.

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