Irish: reduced Anglicized form of Gaelic Ó Duinn, Ó Doinn ‘descendant of Donn’, a byname meaning ‘brown-haired’ or ‘chieftain’. English: nickname for a man with dark hair or a swarthy complexion, from Middle English dunn ‘dark-colored’. Scottish: habitational name from Dun in Angus, named with Gaelic dùn ‘fort’. Scottish: nickname from Gaelic donn ‘brown’.
[Dictionary of American Family Names, Oxford University Press]