Donegan is an anglicized surname originating from several different Irish families.
Numerous spelling variations of the surname Donegan exist. Different spellings include Donegan, Donnegan, Doneghan, Donneghan, Donagan, Donnagan, Donnaghan, Dunegan, Dunnegan, O'Donegan, O'Dunnegan, O'Donnaghan, Dongan, Donegin, Donnegin, Donnagen, Donagen, Donnegen, Donegen, Donnigan, Donigan, Dunnican, Dunican, Dunigan, Dunnigan, McDunnigan, McDonegan, Dongane, Dongan, Dongen, Dungan, and many more.
Ó Donnagáin (dim. of donn, brown)
According to Patrick Woulfe's Irish Names and Surnames
"The name of several distinct families in Ireland. They were chiefs of the extensive district of Ara, now the barony of Ara (or Duhara) in the north-west of Co. Tipperary, and of Ui Cuanach, now the barony of Coonagh in Co. Limerick. They are frequently mentioned in the Annals during the 11th and 12th century, but after the Anglo-Norman invasion they began to decline and soon disappeared from history. Their territory in later times was occupied by a branch of the O'Briens, the chief of which was styled Mac I Brien Ara. The O'Donegans of Cork were anciently chiefs of the Three Plains, now the barony of Orrery in the neighborhood of Rathluirc. Their patrimony was granted by King John of England to William de Barry, under the name of Muskerry-Donegan. There were in ancient times, three distinct families of O'Donegans in Ulster, and the name is still extant in that province. The O'Donegans were numerous at the end of the 16th century in the midlands and in North Connacht; and through by no means common, the name is at the present found in all provinces."
- Woulfe, Patrick; "Irish Names and Surnames", page 502. Genealogical Publishing Com, 1993