About two-thirds of the boundary of Mayo is formed by sea, and the coast is very much indented, and abounds in picturesque scenery. The principal inlets are 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 islands are very numerous, the principal being Inishturk (area 1445 acres, and population 132 in 1881), near Killary Harbour; Clare Island (area 3949 acres, population 62) at the mouth of Clew Bay, where there are many islets all formed of drift; and Achil (area 35,838 acres, population 5070), the largest island in Ireland. In the eastern half of the county, where Carboniferous rocks prevail, the surface is comparatively level, with occasional hills consisting chiefly of granite and slate. The western half is very mountainous, but there are a few valleys adjoining the sea shore. A great portion of the coast extending from Killala to Clew Bay consists of Old Red and Yellow Sandstone. The remainder of the mountainous region consists chiefly of quartzite or alternating beds of quartzite and granite or gneiss schist. Muilrea (2688 feet) is included in a mountain range, lying between Killary Harbour and Lough Mask, which belongs to the Upper Silurian formation. The next highest summits are Nephin (2530 feet), to the west of Lough Conn, and Croagh Patrick (2370 feet), to the south of Clew Bay. The river Moy flows northwards, forming the boundary of the county with Sligo, and falls into Killala Bay. The courses of the other streams are short, and except when swollen by rains their volume is small. The principal lakes are Lough Mask and Lough Corrib, on the borders of the county with Galway, and Loughs Conn, Carrah, Castlebar, Cullin, and Carrowmore. Limestone is abundant, and also iron ore, which, however, is not smelted, from want of fuel. There are several valuable slate quarries; and ochres, granite, and manganese are found.

Agriculture.— There are some very fertile regions in the level portions of the county, but in the mountainous districts the soil is poor, the holdings are subdivided beyond the possibility of affording proper sustenance to their occupiers, and, except where fishing is combined with agricultural operations, the circumstances of the peasantry are among the most wretched of any district of Ireland.

In 1881 there were 179,343 acres, or less than one-eighth of the whole area, under crops, while 545,040 were pasture, 10,702 woods, and 521,673 waste. The total number of holdings in 1881 was 37,693, of which 22,914 were less than 15 acres in extent, and 9386 between 15 and 30 acres. The following table shows the areas under the principal crops in 1855 and 1882 :— [table]

Horses between 1855 and 1882 increased from 17,531 to 18,050, of which 12,150 were used solely for agricultural purposes. The number of cattle in 1855 was 153,583, and in 1882 it was 162,331, of which 53,153 were milch cows. Sheep in 1855 numbered 265,448, and in 1882 only 225,509, although in 1880 there were as many as 271,282. Pigs in 1882 numbered 62,227, goats 5987, and poultry 632,432. According to the latest return the land was divided among 1483 proprietors, who possessed 1,308,367 acres, at an annual rateable value of £310,140, the rateable value per acre being 4s. 8d. The average size of the properties was 882 acres. No fewer than 21 proprietors, possessed upwards of 10,000 acres, and of these 8 possessed upwards of 20,000, viz., Marquis of Sligo, 114,881 acres; Viscount Dillon, 83,749; Sir R. W. H. Palmer, Bart., 80,990; Earl of Lucan, 60,570; T. S. Carter, 37,773; G. Clive, 35,229; Earl of Arran, 29,644; C. H. Knox, 24,374.

Manufactures and Trade.— Coarse linen and woollen cloths are manufactured to a small extent. There are very productive fishing banks on the coast, especially in the neighbourhood of the islands, and the Moy is a fine river for salmon.

Railways.— The Great Northern and Western Railway crosses the county from near Ballyhaunis to Westport, and a branch from it runs north to Ardnaree near Killala Bay.

Administration and Population.— The county includes nine baronies and seventy-three parishes. It is in .the Connaught circuit. Assizes are held at Castlebar, and quarter sessions at Ballina, Ballinrobe, Bellmullet, Castlebar, Claremorris, Swineford, and Westport. There are twenty-two petty sessions districts within the county, and portions of two other districts. It includes the seven poor-law unions of Bellmullet, Castlebar, Claremorris, Killala, Newport, Swineford, and Westport, and part of the unions of Ballina, Ballinrobe, and Castlerea. It is in the Dublin military district, subdistrict of Galway; and there are barrack stations at Castlebar, Ballinrobe, Westport, Foxford, and Ballaghaderreen. In the Irish parliament two inembers were returned for the county, and two for the borough of Castlebar, but at the Union Castlebar was disfranchised.

From 77,508 in 1760 the population gradually increased till in 1841 it was 388,887, but in 1851 it had diminished to 274,499, in 1871 to 246,030, andin 1881 to 245,212, of whom 119,421 were males and 125,791 females. The county contains a portion (with 4318 inhabitants) of the township of Ballina, the townships of Castlebar, 3855, and Westport, 4469, and the town of Ballinrobe, 2286. The number of emigrants from 1st May 1851 to 31st December 1881 was 85,431. The number of emigrants in 1881 was 4469, or a proportion of 18 - 4 to every 1000 of the population. The death-rate to every 1000 of the population for 10 years ending 31st March 1881 was 13 - 8, the birth-rate 27 - l, and the marriage-rate 3'9. Roman Catholics in 1881 numbered 238,262, Episcopalians 5575, Presbyterians 925, and Methodists 275. In 1881 as many as 8808 persons could speak Irish only, and 138,930 Irish and English.

History and Antiquities.— The name given by Ptolemy to the inhabitants of this district of Ireland was Nagnatae. Erris in Μ ayo was the scene of the landing of the chief colony of the Firbolgs, and the battle which is said to have resulted in the overthrow and almost annihilation of this tribe took place also in this county, at Moytura near Cong. Along with the greater part of Connaught it was granted by King John to Hubert de Burgo, but after the rebellion against William de Burgo, third earl, headed by Mac William Oughter, the whole province of Connaught remained nearly independent of British rule till the time of Elizabeth. In the eleventh year of her reign Mayo was made shire ground, taking its name from the monastery of Maio or Mageo, which was the see of a bishop. Even, however, after this period the Mac Williams continued to exercise very great authority. Large confiscations of the estates in the county were made in 1586, on the termination of the wars of 1641, and after the restoration of the Stuarts. Killala was the scene of the landing of a French squadron in connexion with the rebellion of 1798.

There are four round towers in the county,—at Killala, Turlogh, Meelick, and Bal or Ballagh. The monasteries were numerous, and many of them of considerable importance—the principal being those at Mayo, Ballyhaunis, Cong, Ballinrobe, Ballintober, Bur-rishoole, Cross or Holycross in the peninsula of Mullet, Moyne, Rosserick, and Strade. Of the old castles the most notable are Downpatrick, on a cliff 300 feet in height projecting into the sea, Rocktleet near Newport, said to have been built by the celebrated Grace O’Malley, Ballylahan Castle near Foxford, and Deel Castle near Ballina, at one time the residence of the earls of Arran.