"Leitrim, a maritime county of Ireland, province of Connaught, bounded S by Roscommon and Sligo, W by the bay of Donegal, N by Donegal and Fermanagh, and E and SE by Cavan and Longford; 52 m. long and 16 in its greatest breadth, and 6 m. its least, containing 407,260 acres. The surface is extremely uneven, being composed of bogs and high mountains, which afford sufficient herbage for the breeding of cattle. The valleys are fertile, and watered with rivulets, and the mountains contains inexhaustible stores of lead, iron, and copper ores, and coal mines. Potatoes, barley, rye, and wheat, are cultivated in small quantities, and oats in greater abundance. Chief river, the Shannon. 2 members are retutned to parliament. Chief town, Carrick. Pop. 105,976."
[From The New London Gazetteer (1826)]
The Irish Ancestors website (subscription) has the following types of records: State Registration of Births, Marriages & Deaths, Census returns, Land records, Church records, Genealogical Office records, Gravestone inscriptions, Directories, Newspapers, Wills, Deeds, and Occupations.
The Fianna website's pages for County Leitrim provide important addresses and extensive information about online and other genealogy resources.
Roots Ireland (subscription) "offers access to a unique database of more than 20 million Irish records". Its Leitrim coverage includes Baptismal/Birth Records, Marriage Records, Burial/Death Records, Gravestone Inscriptions, Griffith's Valuation (Free Access), and Census Substitutes.