Surnames were not in use in Ireland until about the tenth century. The O'Donnells take their name from Domhnaill, son of Eighneachain (d. 905) and they are sometimes called Clann Dalaigh from Eighneachain's father (d. 874).

The O'Donnells were a leading branch of Cineal Conaill (race of Conall), formed by Conall Gublan who established the Kingdom of Tir Conaill (the county of Conaill) which almost corresponds to the present county of Donegal. Conall Gublan, so called on account of his fosterage or schooling at Binn Gublan (now Ben Bulben) in County Sligo, was a son of Niall of the nine Hostages, the High King of Ireland in the fifth century.

Niall of the Nine Hostages was one of the last pagan High Kings of Ireland. On on of his foreign military expeditions he captured, brought to Ireland and sold as a slave a young boy who afterward became St.Patrick, the Patron Saint of Ireland. Niall was an ancestor of eight of the most illustrious families in Ireland and of the most lasting, most important and most powerful Irish dynasty. His descendants shared the High Kingship, the Cineall Conaill having ten High Kings up to the Anglo-Norman invasion in the twelfth century.

St. Colmcille belonged to the Cineall Conaill, the same family as the O'Donnells and he, too, was close in line for the High Kingship. Other important members of the Cineall Conaill were the O'Cannon's, the Maeldory's, the O'Boyles, the O'Doherty's and the O'Gallagher's.

As leading members of the Cineal Conaill, the O'Donnells were very highly regarded in other countries, as well as Ireland, and were accorded the designation of Princes, Chiefs and Kings of Tir Conaill by the rulers of England, Scotland, France and Spain. Keating, the Irish historian, describes the inauguration ceremony of the O'Donnells as follows - 'the ceremony of inauguration of the Kings of Tir Conaill was thus : The King, being seated on an eminence, surrounded by the nobility and gentry of his own country, one of the chief nobles stood before him, with a straight, white wand in his hand: and on presenting it to the King of Tir Conaill, used to desire him: "To receive the sovereignty of his country, and to preserve equal and impartial justice in every part of his dominions." The reason that the wand was straight and white was to put him in mind that he should be unbiased in his judgment, and pure and upright in all his actions.'