wiktionary
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From Irish Ó Donndubháin "descendant of (a person named) brown + black" + donn ‘brown’, dubh ‘black’ + -án masc. diminutive suffix. The form -áin is the genitive, so ‘of brown-black’
etymonline
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Donovan
surname and masc. proper name, from Irish Donndubhan "dark brown."
