MacDonnell is among the seventy most common names in Ireland and is very numerous in all the provinces. In Ulster it is also well distributed but is most common in counties Antrim and Monaghan. It can be of Scottish or Irish origin.
In Irish Gaelic the name is Mac Domhnaill and outside Ulster the main Irish origin of the name is the MacDonnells, a sept of Thomond in Munster. Most other of the MacDonnells outside Ulster will be descendants of MacDonald galloglasses recorded all over the land from the end of the thirteenth century.
In Ulster, however, there are two origins of the name. In Monaghan, Fermanagh and adjacent areas, most will be descendants of the MacDonnells of Clankelly, Fermanagh's oldest recorded ruling family. When their power was broken by the Maguires they sought refuge in their stronghold of the Connons to the west of Clones and migrated to the MacMahon country of Co. Monaghan, where, in 1300, MacDonnell of Clankelly was recorded as a sub-chief to MacMahon. The last chief, Giolla na Naemh MacDomhnaill was slain in 1501.
For a long time it was thought that the Scottish Clan Donald descended from the Clankelly MacDonnells, but this has now been disproved. The MacDonnells of the Glens of Antrim, through grants for military service as galloglasses and through the the marriage of John Mor MacDonald to Margery Byset of Antrim, carved out an extensive territory and by the sixteenth century had displaced the MacQuillans.
The most famous of this family, Somhairle Buidhe or Sorley Boy MacDonnell, 1505-90, fought successfully for forty years against the O'Kanes, the MacQuillans, the O'Neills and the armies of Queen Elizabeth 1. He annexed the route and penetrated deep into Derry and Tyrone and as far south as Newry in Co. Down. His son Randal MacSorley MacDonnell was created 1st Earl of Antrim in 1620 and from him the present-day Earl of Antrim descends.
(Source 'Book of Ulster Surnames').