The Irish surname Costello is an anglicized form of the Gaelic name Mac Oisdealbhaigh meaning “son of Oisdealbhach,” a personal name composed of the elements os meaning “deer, or fawn” and dealbhach meaning “in the form of, or resembling.” The name is primarily found in the west of Ireland in counties Mayo, Galway and Clare. The family were established in the Barony of Costello in County Mayo where they feuded for centuries with the “MacDermott’s.” The name is first recorded in the New World in 1756 when Honor Costello arrived in New York from Ireland. Today there are approximately 31,000 bearers of the name Costello living in the United States.