[2] The family's eponymous ancestor is Donn Sléibe mac Echdacha, who ruled as king of the Irish petty-kingdom of Dál Fiatach, as well as its over-kingdom, Ulaid, in the late 10th century.
The power-base of the MacDonlevys was Dál Fiatach, a territory which spanned south from the Mourne Mountains north to the River Lagan; at times they exerted control over the entirety of Ulaid (eastern Ulster; present-day counties Down and Antrim).
[2] According to historian C. Thomas Cairney, MacDonlevys were the chiefly family of the Ulaid who were a tribe of the Erainn who were the second wave of Celts to settle in Ireland between about 500 and 100 BC.
As a result, the last-remaining members of the clan migrated to Tír Conaill (present-day County Donegal), whilst others went to Scotland.
[7] The Annals note further that the branch of the MacDonlevy, who had been the physicians, still existed near Kilmacrenan, County Donegal in the early 17th century.