The O'Reilly chiefs were inaugurated on the Hill of Seantoman or Shantoman between Cavan and Ballyhaise, on the summit of which may still be seen the remains of a Druidical temple consisting of several huge stones standing upright and known as Fionn McCool's fingers. In later times they were inaugurated on the Hill of Tullymongan, above the town of Cavan; and took the tribe name of Muintir Maolmordha or the People of Maolmordha, one of their celebrated chiefs. This name Maolmordha or Mulmora was Latinized "Milesius" and anglicised "Miles" or "Myles," - a favorite personal name among the members of the sept. The patron saint of this family was St. Maedoc.

The O'Reillys were for centuries the ruling family of the kingdom of East Breffny, and at their height controlled most of Co. Cavan and large parts of Co. Meath. The family was widely involved in trade in medieval Ireland and at one time 'reilly' was a term for Irish money, subsequently banned by the English. It has also been suggested that they lived well, as the phrase 'the life of Reilly' indicates. They were also notable as ecclesiastics, the family, since the sixteenth century, providing five Primates of All-Ireland, five bishops of Kilmore, two of Clogher and one of Derry.358