O'Donnell is an ancient and powerful Irish clan, lords of Tyrconnel in early times, and the chief rivals of the O'Neills in Ulster. Like the family of O'Neill, that of O'Donnell was descended from Niall of the Nine Hostages, king of Ireland at the beginning of the 5th century; the O'Neill, or Cinel Eoghain, tracing their pedigree to Eogan, and the O'Donnells, or Cinel Conaill, to Conall Gulban, both sons of Niall.

Tyrconnel, the district named after the Cinel Conaill, where the O'Donnells held sway, comprised the greater part of the modern county of Donegal except the peninsula of Inishowen; and since it lay conterminous with the territory ruled by the O'Neills of Tyrone, who were continually attempting to assert their supremacy over it, the history of the O'Donnells is for the most part a record of tribal warfare with their powerful neighbours, and of their own efforts to make good their claims to the overlordship of northern Connacht.

The first chieftain of mark in the family was Goffraidh (Godfrey), son of Donnell Mor O'Donnell (d. 1241). Goffraidh, who was inaugurated as "The O'Donnell", i.e. chief of the clan, in 1248, made a successful inroad into Tyrone against Brian O'Neill in 1252. In 1257 he drove the English out of northern Connacht, after a single combat with Maurice Fitzgerald in which both warriors were wounded. O'Donnell while still incapacitated by his wound was summoned by Brian O'Neill to give hostages in token of submission. Carried on a litter at the head of his clan he gave battle to O'Neill, whom he defeated with severe loss in prisoners and cattle; but he died of his wound immediately afterwards near Letterkenny, and was succeeded in the chieftainship by his brother Donnell Óg , who returned from Scotland in time to withstand successfully the demands of O'Neill.

In the 16th century, when the English began to make determined efforts to bring the whole of Ireland under subjection to the crown, the O'Donnells of Tyrconnel played a leading part; co-operating at times with the English, especially when such co-operation appeared to promise triumph over their ancient enemies the O'Neills, at other times joining with the latter against the English authorities.

The Cinel , or Kinel , was a group of related clans occupying an extensive district. See P. W. Joyce , A Social History of Ireland (London, 1903), i. 166.