Mayo, county (1991 pop. 110,696), 2,084 sq mi (5,398 sq km), W Republic of Ireland. The county seat is Castlebar. The western portion, including large Achill island, is mountainous; the eastern part is more level. There are numerous lakes (Mask, Carrowmore, Cullen, Conn, and Carra), and the irregular coast line is deeply indented by bays (Killala, Broadhaven, Blacksod, and Clew). Oats and potatoes are grown; cattle, sheep, pigs, and poultry are raised. Bacon curing, woolens manufacturing, and flour milling are carried on. Tourism is developing. The region was granted to the De Burghs after the Anglo-Norman invasion of Ireland, but the county was not brought fully under English control until the late 16th cent.