From Atlas and Cyclopedia of Ireland (1900)
NAME.—The county took its name from the village of Leitrim, near the Shannon, 4 miles above Carrick-on-Shannon. The Gaelic form of the name is Liath-druim (pron. Leedrim), signifying gray ridge (liath, grey; druim, a ridge or long hill); and there are more than forty places of the name in Ireland.
SIZE AND POPULATION.—The county consists of two parts, almost wholly separated from one another by Lough Allen. The northwest part touches the sea, having a coast of 2 ½ miles on Donegal Bay. The greatest length of the two parts taken together, from Donegal Bay to the southern extremity near Drumlish in Longford, is 51 miles; breadth of the northwest part, from the boundary near Ballintogher in Sligo to Upper Lough Macnean, 17 miles; breadth of the southeast part, from Lough Boderg to the boundary near Killygar, 18 miles; area, 613 square miles; population, 90,372.
SURFACE.—The northern half of the county is all mountainous or hilly, with the exception of a narrow east-and-west belt extending in breadth from Donegal Bay to Lough Melvin and the river Duff. The north part of the other half, viz., that part east of Lough Allen, is mountainous, being occupied by a portion of that mountain group that covers also the northwest projection of Cavan. The south part, viz., the barony of Mohill, and the southern portions of the baronies of Carrigallen and Leitrim, is moderately level, but in many places it is interrupted by low heights and ridges.
MOUNTAINS AND HILLS.—The most remarkable mountain in the whole county is Slieve Anierin (1,922), whose summit is 2 ½ miles east of the shore of Lough Allen; a little northeast of which is Bencroy (1,707). Slievenakilla (1,793), east of the head of Lough Allen, stands on the boundary with Cavan. In the northwest portion of the county there is an endless succession of summits of all heights up to 1,700 feet. Two miles west of Manorhamilton is the conspicuous mountain of Benbo (1,365). The summit of Truskmore (2,113) is in Sligo, but a part of its eastern slope extends into Leitrim.
RIVERS.—The Shannon, coming from Cavan, forms the boundary for a mile and a half; then crossing the narrow neck connecting the two parts of Leitirm for another mile and a half, it enters Lough Allen; and from that down to a point a little below Roosky, a distance of about 35 miles (following the larger windings) it forms the western boundary of the county. On the northeast, the stream flowing from Upper Lough Macnean to Lough Melvin—called the Kilcoo River in the lower half of its course—forms the boundary between Leitrim and Fermanagh. The river Drowes has a course of 4 miles from Lough Melvin to Donegal Bay, the first mile of which is in Leitrim, and the last three is the boundary between Leitrim and Donegal. This little river is mentioned in Gaelic records as having from the most ancient times separated Connaught from Ulster, and it still continues the boundary between the two provinces. The Kilco River receives the Lattone from the Leitrim side; and near it on the west are the Ballagh River and Glenaniff River, both flowing into the head of Lough Melvin. North of Lough Melvin, the Bradoge, flowing to the west from Fermanagh, forms for 2 miles the boundary between Leitrim and Donegal, after which it enters Donegal. In the extreme northwest the Duff (called the Black River in the early part of its course), forms the boundary between Leitrim and Sligo for 2 miles; then crosses Leitrim for 2 miles; and lastly, forms again the boundary between the same two counties for a mile, till it enters Donegal Bay. South of this the Diffreen runs west into Glencar Lake.
The Bonet rises in Glenade Lake, in the barony of Rosclogher, and flows first southeast through Glenade, one of the most beautiful valleys in the whole district; then gradually curving, it passes by Drumahaire and falls into Lough Gill, flowing through a succession of lovely landscapes through its whole course. The Owenmore or Seardan passes through Manorhamilton, and falls into the Bonet a mile below the town.
To the north of Lough Allen the Owenayle, flowing southward, forms the eastern boundary (between Leitrim and Cavan) for 4 ½ miles till it falls into Shannon. The Yellow River rises in the glens between Bencroy and Slievenakila, and flows westward into Lough Allen; and the Stony River runs down the side of Slieve Anierin into the same lake. On the west side, Lough Allen receives the Diffagher River and the Owengar, which unite and flow into the northwest corner of the lake. The Arigna flows to the southeast for several miles on the boundary between Leitrim and Sligo, after which it enters the county Roscommon, and ultimately falls into the Shannon where it issues from Lough Allen. Southeast of Lough Allen, the Aghacashlaun flows southward down the slopes of Bencroy Mountain and into Lough Scur, the overflow of which is poured into the Shannon at the village of Leitrim. Near this on the east, the Yellow River flows south and east, by the village of Ballinamore into Garadice Lough.
LAKES.—Leitrim, like the neighboring counties of Fermanagh, Cavan, and Roscommon, is dotted all over with lakes. Lough Allen, in the middle (a small part of which belongs to Roscommon), is 8 ½ miles long and 3 miles broad at its north or widest end. It is nearly surrounded with hills, so that it occupies the bottom of a basin, down the slopes of which rivers pour into the lake from every side.
The following lakes lie round the margin of the county, beginning on the north and going from left to right: Lough Melvin and Upper Lough Macnean have been spoken of in Fermanagh; Derrycassan Lake (part of which belongs to Cavan), from which the Woodford River in Cavan issues; Glasshouse Lake, also on the boundary with Cavan. Passing over several small lakes we come to those on the Shannon, viz., Lough Bofin and Lough Boderg. Lastly, Lough Gill, Glencar Lake, and Cloonty Lake, all which are mentioned in Sligo.
The chief lakes in the interior are: in the north part of the county the lovely Glenade Lake, a little over a mile in length, occupying the head of a fine valley, which is traversed by the Bonet River issuing from the lake. The small lake of Munakill lies near Manorhamilton; and the larger lake of Belhavel is east of Drumahaire. In the interior of the southern part of the county, Garadice Lough, or Lough Finvoy, a very beautiful sheet of water, 2 ½ miles in length, lies near the east margin. Lough Rinn, near Mohill, is 3 miles in length; Lough Scur, a mile and a half long, and the smaller lake of Carrickaport, both lie southeast of Drumshambo; east of these is the irregularly shaped St. John's Lake, about 2 miles in length. The small lakes scattered over this southern portion of the county are numerous beyond description.
TOWNS.—Carrick-on-Shannon (1,384), the assize town, Mohill (1,117), and Ballinamore (526), are all in the southern division of the county. In the center of the northern division is Manorhamilton (1,225), standing in the midst of a lovely country; and at the south corner of Lough Allen is Drumshanbo (544).
MINERALS.—Lough Allen occupies the center of the great Connaught coal district, a considerable portion of which belongs to Leitrim. There are coal pits in several places round the lake, especially at and near Slieve Anierin, the coal being raised for smelting purposes. What is called the Arigna iron district belongs partly to Leitrim, and partly to the county Roscommon. Iron ore abounds on Slieve Anierin, and the mines were worked for a long period. The very name of the mountain shows that the presence of iron was known ages ago, when the name was imposed; for Slieve-an-ierin signifies the "Mountain of iron."
ANCIENT DIVISIONS AND DESIGNATIONS.—This county was formerly called Brefny O'Rourke; it was the principality of the O'Rourkes, and from the same family the village of Drumahaire was often called Bally-O'Rourke. Brefny O'Rourke included also a part of the northwest extremity of Cavan. The barony of Rosclogher was formerly, and is still, known by the name of Dartry; and was possessed by the family of Mac Clancy. The southern or level part of the county, the territory of the Mac . Rannalls, or Reynolds, was called Moy Rein, and often Munter Eolais.