"Monaghan, a county of Ireland, province of Ulster, bounded N by Tyrone, E by Armagh, and SE by East Meath and Louth, 30 m. long, and 19 broad, and containing 288,500 English acres, or 450 square miles, divided into 21 parishes. Although it is much encumbered with bogs and mountains, great portions of it are highly cultivated and improved. the linen manufcture, which flourishes extensively, is the great staple. The Sleibh-Baught mountains, which extend into this county from Tyrone, though they are an uninteresting waste, have, however, beds of the richest limestone, and abundance of marl; and indications of coal have also been observed; also lead ore of the highest quality. This county contains 184 lakes, of which 30 are considerable sheets of water. Principal rivers, the Ballyhays, Annalee, Lagan, Fane, Myrvale, Fin, and Blackwater. Pop. 178,183."
[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 Monaghan 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 Monaghan coverage includes Baptismal/Birth Records, Marriage Records, Burial/Death Records, Census Records, Gravestone Inscriptions, Griffith's Valuation (Free Access), and Census Substitutes.