LOUTH, a maritime county in the province of Leinster, Ireland, bounded N.E. by Carlingford Bay and Co. Down, E. by the Irish Sea, S.W. by Meath, and N.W. by Monaghan and Armagh. It is the smallest county in Ireland, its area being 202,731 acres or about 317 sq. m. The greater part of the surface is undulating, with occasionally lofty hills; in the north-east, on the borders of Carlingford Lough, there is a mountain range approaching 2000 ft. in height. Many of the hills are finely wooded, and towards the sea the scenery, in the more elevated districts, is strikingly picturesque. With the exception of the promontory of Clogher Head, which rises abruptly to a height of 180 ft., the coast is for the most part low and sandy. The narrow and picturesque Carlingford Lough is navigable beyond the limits of the county, and Carlingford and Greenore are well-known watering-places on the county Louth shore. The Bay of Dundalk stretches to the town of that name and affords convenient shelter. The principal rivers, the Fane, the Lagan, the Glyde and the Dee, flow eastwards. None of these is navigable, but the Boyne, which forms the southern boundary of the county, is navigable for large vessels as far as Drogheda.
Almost all this county is occupied by an undulating lowland of much-folded Silurian shales and fine-grained sandstones; but Carboniferous Limestone overlies these rocks north and east of Dundalk. Dolerite and gabbro, in turn invaded by granite, have broken through the limestone north of Dundalk Bay, and form a striking and mountainous promontory. There is now no doubt that these rocks, with those on the adjacent moorland of Slieve Gullion, belong to the early Cainozoic igneous series, and may be compared with similar masses in the Isle of Skye. A raised beach provides a flat terrace at Greenore. Lead ore has been worked in the county, as in the adjacent parts of Armagh and Monaghan.
In the lower regions the soil is a very rich deep mould, admirably adapted both for cereals and green crops. The higher mountain regions are covered principally with heath. Agriculture generally is in an advanced condition, and the farms are for the most part well drained. The acreage of tillage is but little below that of pasture. Oats, barley, flax, potatoes and turnips are all satisfactorily cultivated. Cattle, sheep, pigs and poultry represent the bulk of the live stock. Linen manufactures are of some importance. The deep-sea and coast fishery has its headquarters at Dundalk, and the salmon fisheries at Dundalk (Castletown river) and Drogheda (river Boyne). These fisheries, together with oyster beds in Carlingford Lough, are of great value. The county is traversed from S. to N. by the Great Northern railway, with a branch westward from Dundalk; while the same town is connected with the port of Greenore by a line owned by the London & North-Western railway of England. From Greenore the London & North-Western railway passenger steamers run regularly to Holyhead. The town of Ardee is served by a branch from the Great Northern line at Dromin.
The population (71,914 in 1891; 65,820 in 1901) decreases at about an average rate, and a considerable number of the inhabitants emigrate. Of the total population about 92% are Roman Catholics. The principal towns are Dundalk (pop. 13,076), Drogheda (12,760) and Ardee (1883). The county includes six baronies and sixty-four parishes. Assizes are held at Dundalk and quarter sessions at Ardee, Drogheda and Dundalk. Louth was represented by two county and ten borough members in the Irish parliament; the two present divisions are the north and south, each returning one member. The county is in the Protestant dioceses of Armagh and Clogher and the Roman Catholic diocese of Armagh.
The territory which afterwards became the county Louth was included in the principality of Uriel, Orgial or Argial, which comprehended also the greater part of Meath, Monaghan and Armagh. The chieftain of the district was conquered by John de Courcy in 1183, and Louth or Uriel was among the shires generally considered to have been created by King John, and peopled by English settlers. Until the time of Elizabeth it was included in the province of Ulster. County Louth is rich in antiquarian remains. There are ancient buildings of all dates, and spears, swords, axes of bronze, ornaments of gold, and other relics have been discovered in quantities. Among Druidical remains is the fine cromlech of Ballymascanlan, between Dundalk and Greenore. Danish raths and other forts are numerous. It is said that there were originally twenty religious houses in the county. Of the remains of these the most interesting are at Monasterboice and Mellifont, both near Drogheda. At the former site are two churches, the larger dating probably from the 9th century, the smaller from the 13th; a fine round tower, 110 ft. in height, but not quite perfect; and three crosses, two of which, 27 and 15 ft. in height respectively, are adorned with moulding, sculptured figures and tracery, and are among the finest in Ireland. At Mellifont are the remains of the first Cistercian monastery founded in Ireland, in 1142, with a massive gatehouse, an octagonal baptistery and chapter-house. Carlingford and Drogheda have monastic remains, and at Dromiskin is a round tower, in part rebuilt. Ardee, an ancient town, incorporated in 1376, has a castle of the 13th century. At Dunbar a charter of Charles II. (1679) gave the inhabitants the right to elect a sovereign. Louth, 512 m. S.W. from Dundalk, is a decayed town which gave its name to the county, and contains ruins of an abbey to which was attached one of the most noted early schools in Ireland.