MAYO, a western county of Ireland, in the province of Connaught, bounded north and west by the Atlantic Ocean, north east by Sligo, east by Roscommon, south-east and south by Gal way. The area is 1,380,390 acres, or about 2,157 sq.m., the county being the third largest in Ireland. Pop. (1926) 172,661.
The wild and barren west of the county, including the great hills on Achill Island, is formed of "Dalradian" rocks, schists and quartzites, highly folded and metamorphosed, with intrusions of granite near Belmullet. At Blacksod Bay the granite has been quarried as an ornamental stone. Nephin Beg, Nephin (2,646 ft.) and Croagh Patrick (2,510 ft.) are typical quartzite summits, the last named belonging possibly to a Silurian horizon but rising from a metamorphosed area on the south side of Clew Bay. The schists and gneisses of the Ox Mountain axis also enter the county north of Castlebar. Muilrea to the north of the fjord of Killary Harbour, reaches a height of 2,688 ft. To the east of Lough Mask is a Carboniferous Limestone plain. Silurian rocks, with Old Red Sandstone over them, come out at the west end of the Curlew range at Ballaghaderreen. Clew Bay, with its islets capped by glacial drift, is a submerged part of a synclinal of Carboniferous strata, and Old Red Sandstone comes out on the north side of this, from near Achill to Lough Conn. The country from Lough Conn northward to the sea is a lowland of Carboniferous Lime stone, with L. Carboniferous Sandstone against the Dalradian on the west.
The coast is much indented, the principal inlets being Killary Harbour between Mayo and Galway; Clew Bay, in which are the harbours of Westport and Newport; Blacksod Bay and Broad Haven, which form the peninsula of the Mullet; and Killala Bay between Mayo and Sligo. The principal islands are Inishturk, near Killary Harbour; Clare Island, at the mouth of Clew Bay, where there are many islets, all formed of drift; and Achill, the largest island off Ireland. The river Moy flows northwards, form ing part of the boundary of the county with Sligo, and falls into Killala Bay. The principal lakes are Lough Mask and Lough Corrib, on the borders of the county with Galway, and Loughs Conn in the east, Carrowmore in the north-west, Beltra in the west, and Carra adjoining Lough Mask.
Erris in Mayo was the scene of the landing of the chief colony of the Firbolgs, and Moytura near Cong saw their overthrow and almost complete annihilation. At the close of the i 2th century what is now the county of Mayo was granted by king John to William, brother of Hubert de Burgh. In the 14th century the
land passed to a branch of the family known as "MacWilliam Oughter." Mayo was made shire ground during the first viceroy alty of Sir Henry Sydney, taking its name from the monastery of Maio or Mageo, which was the seat of a bishop. Even after this period the MacWilliams continued to exercise authority, which was regularized in 1603, when "the MacWilliam Oughter," Theo bald Bourke, surrendered his lands and received them back, to hold them by English tenure, with the title of Viscount Mayo. (See BURGH, DE.) Large confiscations were made in 1586, and on the termination of the wars of 1641; and in 1666 the restoration of his estates to the 4th Viscount Mayo involved another con fiscation, at the expense of Cromwell's settlers. Killala was the scene of the landing of a French squadron in connection with the rebellion of 1798.
There are round towers at Killala, Turlough, Meelick and Balla, and an imperfect one at Aughagower. Killala was formerly a bishopric. The principal monasteries were those at Mayo, Bally haunis, Cong, Ballinrobe, Ballintober, Burrishoole, Cross or Holy cross in the peninsula of Mullet, Moyne, Roserk or Rosserick and Templemore or Strade. The most notable old castles are Car rigahooly near Newport, and Deel Castle near Ballina.
The thin soil of the mountains barely supports a scanty popu lation. Conditions are better in the valleys and along the coasts where the fisheries are important. Oats and potatoes are the principal crops. Cattle, sheep, pigs and poultry are reared. Coarse linen and woollen cloths are manufactured to a small extent. At Foxford woollen-mills are established at a nunnery, in connection with a scheme of technical instruction. Keel, Belmullet and Ballycastle are the headquarters of sea and coast fishing districts, and Ballina of a salmon-fishing district.
Claremorris is an important centre on the Great Southern rail way. The line from Athlone runs through it and continues north to Ballina and Killala. Similarly it is served by the line from Athenry to Sligo, and has a branch to Ballinrobe. There is a branch from Manulla to Westport and Achill. North Mayo, con sisting of the county electoral areas of Ballina, Killala and Swin ford, returns 4 members to Dail Eireann. South Mayo, consisting of the county electoral areas of Castlebar, Claremorris and West port returns 5 members.