The Irish surname O'Hagan has two possible origins. The name may be an anglicized form of Gaelic Ó hÁgáin “descendant of Ógán,” a personal name from a diminutive of óg meaning “young.’ The name may also be an anglicized form of Gaelic Ó hAodhagáin ‘descendant of Aodhagán,” a personal name formed from a double diminutive of Aodh meaning “fire.” The great O'Hagan clan originated in the province of Ulster, and the seat of its chief was at Tullahogue, County Tyrone. The chief exercised the hereditary right of inaugurating O'Neill as king or overlord of Ulster. In medieval times, members of the sept were territorial magnates in Counties Monaghan and Armagh, and two places called Ballyagan, (from "baile", a settlement), one in County Derry and the other in Antrim, further locate the O'Hagans. Among the several notable bearers of the name were Thomas O'Hagan, (1812-1885), Lord Chancellor of Ireland.