Wikipedia page on the O’Carroll name states:
“Notable is the history of the Clan Ó Cearbhail whose territory, known as Ely O’ Carroll in Éile, consisted of the pasture lands of Ballycrinass, Rosscullenagh and Drumcan, extending to the Lake of Leghagh, commonly Laghaghirisallive and bounded on the west by the lands called Laghenagarken and on the east adjoining or near to Glencrokin. This was always known as Ely O’ Carroll. The mountain land extended from the Laghanagerah (Lochan na gCaorach) to Polle Dowa (Poll Dubh in Roscomroe) and then in a south easterly direction to the Slieve Bloom Mountains, which are the limits between Ely O’ Carroll and Upper Ossary meeting at a village called Garryvoe or Scully’s land.
The Ely O’ Carroll sept was the most powerful and most famous,but there were at least four other septs, O’ Carrolls of Oriel in the Dundalk area, O’ Carrolls of Ossory in the Carlow/Kilkenny area and two septs in the Kerry area. The prefix “O'” is very often dropped in the case of this surname, giving simply Carroll, occasionally replaced with “Mac”.”
The Birr Historical Society website (www.birrhistsoc.com) outlines the huge influence and power that the Ely O’Carroll Sept held within Ireland:
“Most of the territory of South Offaly was once known as Ely O’Carroll or Eile Ui Chearbhaill which meant Eile of the O’Carrolls. The UíChearbhaill (O’Carrolls) were a formidable dynasty, a branch of a more ancient people called the Éile whose territory came to be divided between the Uí Fhógartaigh (O’Fogartys) and the Uí Chearbhaill (O’Carrolls). The petty kingdom of Éile Uí Chearbhaill was ruled for many centuries by the descendants of Cearbhall, overlord of Éile Tuaisceart in the second half of the tenth century AD.
The O’Carrolls are associated with many castles in South Offaly and North Tipperary and resisted domination by the Anglo Normans and other powerful neighbours. Tadgh O’Carroll, commissioned the decoration of the shrine for the Book of Dimma in the late fourteenth century and was killed at the battle of Callan in 1470 A.D. His daughter Margaret O’Carroll, married Calvagh O’Connor Faly. She was a celebrated patroness of culture and the benefactress of large assemblies of literati and alms-seekers at Killeigh and Rathangan during the hungry summer of 1433. She made a pilgrimage to Compostella in Spain in 1445.
The O’Carrolls intermarried with the Fitzgeralds of Kildare, the O’Connors Fály, the O’Briens of Thomond, the Butlers of Ormond and other powerful chieftain families. In the sixteenth century, the wife of Ferganainm O’Carroll was the daughter of Garret Óg Fitzgerald and the sister of Silken Thomas. The English policy of ‘Surrender and regrant’ in the sixteenth century put the O’Carrolls under considerable pressure at a time when they were racked by internal feuds and sometimes at war with other Irish septs. Birr Castle was one of their chief strongholds. The O’Carrolls lost power steadily in the seventeenth century. Ely O’Carroll was shired and added to the King’s County (now Co. Offaly) in 1605 and the O’Carroll lands were confiscated for the Jacobean and Cromwellian plantations.”
Some of the dispossessed branches of the family remained in Ireland after the plantations, others went abroad. Charles O’Carroll, one of the signatories of the American Declaration of Independence in 1776, was descended from his Irish namesake Charles O’Carroll from the Letterluna branch who left Ireland and received a large grant of land in 1688 in Maryland, U.S.