The name Reilly originated from the Irish Gaelic surname Ó Raghailligh, which means "descendant of Raghaillach". Raghaillach is derived from the Old Irish word "ragallach", meaning "arrogant" or "defiant". The name first gained popularity in Ireland during the Middle Ages.

The name Reilly can be traced back to the ancient Gaelic clans of Ireland. It was particularly prominent among the families of County Cavan and County Westmeath. Records show that the name appeared in various medieval Irish manuscripts and annals, indicating its widespread usage among the Irish nobility and chieftains.

One of the earliest recorded instances of the name Reilly dates back to the 12th century. Reilly Óg Ó Raghailligh, a prominent Irish chieftain, is mentioned in the Annals of the Four Masters, a renowned chronicle of medieval Irish history. He was a member of the powerful Ó Raghailligh clan and played a significant role in local affairs during his time.

Throughout history, several notable individuals have borne the name Reilly. One of the most famous was John Reilly (1580-1628), an Irish Franciscan friar and martyr who was hanged, drawn, and quartered during the English Reformation for his Catholic beliefs. Another prominent figure was Sir John Reilly (1839-1912), an Irish-American businessman and philanthropist who played a crucial role in the development of the city of Indianapolis.

In the realm of sports, Johnny Reilly (1917-1977) was a professional baseball player who spent his entire career with the New York Yankees, winning four World Series championships. More recently, John C. Reilly (born 1965) is an acclaimed American actor known for his versatile roles in films such as "Chicago," "Walk Hard: The Dewey Cox Story," and "Step Brothers."

Edwina Reilly (born 1954) is a notable Irish writer and journalist who has authored several novels and non-fiction works, including her memoir "Prefabulous" and the novel "Growing Pains." Her writing often explores themes of family, identity, and Irish culture.

While the name Reilly has its roots in Ireland, it has gained popularity worldwide and transcended cultural boundaries. Its rich history and enduring presence in various fields have contributed to its recognition and appeal across generations.