"Mayo, a county of Ireland, province of Connaught, 62 m. long and 52 broad, bounded E by Roscommon, S by Galway, W and N by the Atlantic, and NE by Sligo, 54 m. long, and 48 broad, and containing 796,000 acres, divided into 68 parishes. It sends 2 members to parliament. The W coast is mountainous, and thinly inhabited; but the interior produces excellent pasturage, and is watered by several lakes and rivers. Pop. 297,538. Chief town, Castlebar."
[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 Mayo 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 Mayo coverage includes Baptismal/Birth Records, Marriage Records, Burial/Death Records, Census Records and Griffith's Valuation (Free Access).