East Breifne

The Kingdom of East Breifne or Breifne O'Reilly (Old Irish: Muintir-Maelmordha; Irish: Bréifne Uí Raghallaigh, IPA:) was an historic kingdom of Ireland roughly corresponding to County Cavan that existed from 1256 to 1607. It took its present boundaries in 1579 when East Breifne was renamed Cavan, after Cavan town, and shired into Ulster.

Originally part of the older Kingdom of Breifne, East Breifne emerged following a protracted war between the ruling O'Rourke clan and the ascendant O'Reillys, culminating in the kingdom's division in 1256. The Kingdom was ruled by the Ó Raghallaigh (O'Reilly) dynasty and lasted until the early 17th century.