The McDonnells of Knocknacloy are a branch in Ireland of the Highland Scottish clan, Clan Donald. The McDonnells of Knocknacloy are descended from Somerled Macdonald son of Alastair Óg MacDomhnaill and Juliana MacDougall.
They were Gallóglaigh who offered their services to the O'Neills of Tyrone and fought against their cousins the MacDonnell of Antrim and settled at Knocknacloy in County Tyrone.
References
McDonnells of Knocknacloy Wikipedia(Text) CC BY-SA