The Uí Laoghaire clan, today associated with the Uibh Laoghaire parish in County Cork, is considered by scholars to have originated in the early Middle Ages on the south-west coast, in the area of Ros Ó gCairbre (Rosscarbery), of which the O'Leary were hereditary lords. The book of Lecan (A.D. 1397-A.D. 1418) details the early status of O'Leary as a Corco Laide taisach duchusa (hereditary chieftain) in the tuath of Ross (Ruis), with associated families: