The Irish surname Geary is an anglicised rendering of the Gaelic surname O'Gadhra, also translated as O'Gara. This name denotes one who is a "descendant of Gadhra", an ancient chief whose name itself may be traceable to the Old Irish term "gadhar", meaning "hound".

The clan was based in County Sligo and spread to West Munster and North Connacht in the 15th century before spreading around the world. The Gearys were prominent in the Catholic Church and in the education of clergy.