"Roscommon, a county of Ireland, province of Connaught, bounded E by Longford and Westmeath, S by Galway, W by Mayo, N by Sligo, and NE by Leitrim, 50 English m. long, and 37 broad. It is a flat open coutry, in some places sprinkled with rocks, in many interrupted by extensive bogs, and but little diversified by hills. The Shannon divides it from Longford and Westmeath, and the Suck from Galway. It has excellent pastures. Pop. 207,777."
[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 Roscommon 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 Roscommon coverage includes Baptismal/Birth Records, Marriage Records, Burial/Death Records and Griffith's Valuation (Free Access).