Donnelly is an Irish surname. It is the Anglicized form of the Gaelic "Ó Donnghaile", "O" meaning male descendant of ("Ní" is the female equivalent), and Donnghaile (or Donnghal), a personal name composed of the elements "donn" (brown), plus "gal" (valour). The name O’Donnelly is derived from the descendants of Donnghaile (Donnghal) (an individual who lived during 10th century in Ireland) who was the great grandson of Domhnall, King of Aileach. Early ancestors of this surname were a part of Cenél nEoghain and the Uí Néill as descendants from the line of Eógan mac Néill one of the seven sons of Niall Noígíallach.
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It is most commonly found in County Tyrone in mid- and west Ulster. It was so common in Roscommon that at some point, a branch of people with the Donnelly surname took on the name of Briscoe instead.