Castlebar is the county town of County Mayo, Ireland. Developing around a 13th-century castle of the de Barry family, from which the town got its name, the town now acts as a social and economic focal point for the surrounding hinterland. With a population of 13,054 in the 2022 census, Castlebar was one of the fastest growing towns in Ireland in the early 21st century.

County Mayo is a county in Ireland. In the West of Ireland, in the province of Connacht, it is named after the village of Mayo, now generally known as Mayo Abbey. Mayo County Council is the local authority. The population was 137,231 at the 2022 census. The boundaries of the county, which was formed in 1585, reflect the Mac William Íochtar lordship at that time.