Mayo, a maritime county of the province of Connaught, Ireland, is bounded on the N. and W. by the Atlantic Ocean, E. by Sligo and Roscommon, and S. by Galway. The greatest length north and south is 68 miles; the greatest breadth, 57 miles. Area, 1,360,731 acres, of which nearly 26 per cent. is bog and 18 per cent. barren. Of the remainder about 170,000 acres are cropped, cereals covering from 50,000 to 60,000, potatoes and other green crops somewhat more, the rest being in pasture. Pop. (1841) 388,887; (1861) 254,769; (1881) 245,212, of whom 238,262 were Roman Catholics; (1891) 219,034. The rearing of cattle and agriculture are the leading industries. The eastern half of the county is more or less a plain, the western half mountainous, the highest points being Mulrea (2688 feet), Nephin (2530), and Croagh Patrick (2370). Ironstone abounds, but owing to want of fuel is not worked; and there are several valuable slate-quarries. The chief towns are Castlebar (3885), Westport (4469), Ballina (5760, including 1442 in County Sligo), and Ballinrobe (2286). The coast-line of Mayo is about 250 miles, and is greatly indented, Killala, Blacksod, and Clew Bays, Killary Harbour, and Broad Haven being on this coast. Off Mayo, too, lie the islands Achil (35,283 acres), Clare (3959), and others. Loughs Mask and Corrib lie on the southern border, and Loughs Conn, Castlebar, Cullen, Carragh, Corramore within the county. A valuable salmon-fishery exists in the river Moy, and Lough Mask is the home of the 'gillaroo' trout. The Irish language was in 1881 spoken by 8808 persons who did not know English, and by 138,930 who did. Four members are returned to the House of Commons.
Mayo formed part of the extensive territory granted by King John to Hubert de Burgh; but William, the third earl, seizing Galway and Mayo, threw off the English allegiance and adopted the 'customs of the Irish,' together with the Celtic name of Mac-William. The district was not subdued until 1586. The antiquities of Mayo are chiefly ecclesiastical, there being many ruins of monasteries. Four round towers exist, and at Cong the remains of a splendid abbey of the 12th century. The celebrated 'Cross of Cong,' now in the museum of the Royal Irish Academy, was made at Roscommon in 1120.