The name Cassidy originated from the Irish Gaelic name "Cáisídhe", which is derived from the word "cáis", meaning "clever" or "ingenious". It was initially used as a surname in Ireland, particularly in the counties of Fermanagh and Tyrone.

The earliest recorded use of the name Cassidy dates back to the 16th century, when it was documented in the Annals of Ulster, an ancient Irish chronicle. During this period, the Cassidy clan was prominent in Ulster, and the name was closely associated with the region.

One of the earliest notable individuals with the name Cassidy was Felim Ó Cáisídhe (c. 1560-1631), an Irish chieftain and leader of the Cassidy clan in Fermanagh. He played a significant role in the Nine Years' War (1593-1603) against English forces.

In the 17th century, the name Cassidy gained popularity as a given name, particularly among Irish Catholic families. One notable figure was Father Patrick Cassidy (1647-1684), an Irish Catholic priest and martyr who was executed during the Popish Plot, a fictitious conspiracy against King Charles II.

As the Irish diaspora spread across the world, the name Cassidy traveled with them. In the late 19th century, Butch Cassidy (Robert LeRoy Parker, 1866-1908) became one of the most famous outlaws in American history, leading the Wild Bunch gang in the American Old West.

Another prominent figure with the name Cassidy was Jack Cassidy (1927-1976), an American actor and singer known for his roles on Broadway and television shows like "He & She" and "The Eiger Sanction". He was also the father of actress Shaun Cassidy and the late actor David Cassidy.

In the world of literature, Eva Cassidy (1963-1996) was an American singer and guitarist renowned for her unique interpretations of jazz, blues, and folk songs. Although she gained little recognition during her lifetime, her posthumous album "Songbird" became a critical and commercial success.

Cassidy has also been a popular name in sports, with notable figures including Cassidy Hubbarth, an American sportscaster and host for ESPN and ABC, and Cassidy Darling, an American Olympic swimmer who competed in the 2012 Summer Olympics in London.