The name O'Carroll is derived from Cearbhaill, (Irish "Warrior Champion" or, more accurately rendered, "Slaughter"), and Son of Aedh, who was Chief of the Cianachta and King of Eile. Prince Cearbhaill led the Eilians for King Brian Boru at the Battle of Clontarf 1014. Since 1983, Eile F.J. O'Carroll, has become the recognized Chief of Name, and is a member on the Standing Council of Irish Chiefs and Chieftains.
After the Norman invasion, the Butler family, originally known as Fitz Walter, gained control of much of Ormond. The O'Carrolls maintained dominion of a area of about 300,000 acres, today known as the Eile O'Carroll Territory, up until the Cromwellian Invasion and the collapse of the Gaelic Order in the 1600's. It consisted of eight tuathas or baronies, each ruled by a chief subordinate to the Eile O'Carroll sept. It is centered around the towns of Birr in County Offaly and Roscrea in County Tipperary, extending north to above Banagher and south-west along Lough Derg to below Nenagh, then east to the Slieve Bloom Mountains. This is the very Heart of Ireland.