Mac Giolla Dhuibh is an Irish surname derived from the Gaelic phrase meaning "son of the black-haired man." Composed of the elements mac "son," giolla "youth" or "devotee," and dhuibh the genitive of dubh "black," this name refers to a physical characteristic of an ancestor—likely dark hair akin to the pre-Norman Irish tradition of descriptive epithets. The Anglicized version of this surname is Kilduff, which became more common after the decline of Gaelic culture in the 17th-19th centuries as administrative and legal pressures encouraged English forms. Sporadic recording suggests Kilduff concentrates in counties Galway and Roscommon. Meaning: "Son of the black-haired man" Origin: Irish Gaelic (pre-10th century patronymic) Type: Surname Usage Regions: Western Ireland primarily, particularly Connacht