"Westmeath, a county of Ireland, province of Leinster, bounded N by Cavan, E by Eastmeath, S by King's county, NW by Longford, and W by Roscommon, 33 Irish m. long from E to W and 27 broad, contains 249,943 acres, Irish plantation measure, including bogs, mountain, and waste. It is agreeably interspersed with beautiful lakes. The chief rivers are the Shannon, Inny, and Brosna. It returns 3 members to the Imperial parliament. Pop. 128,042."
[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 Westmeath 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 Westmeath coverage includes Baptismal/Birth Records, Marriage Records, Burial/Death Records, Gravestone Inscriptions, Griffith's Valuation (Free Access), and Census Substitutes.