"Londonderry, a county of Ireland, province of Ulster, 40 m. long and 31 broad, bounded W by Donegal, N by the ocean, S and SW by Tyrone, and E by Antrim, containing 511,688 English acres, divided into 31 parishes. It sends 4 members to parliament. 1/4 seems to be bleak mountains and glens, and 1/2 of the remainder is mountainous. Potatoes are the principal crop; and in some parts oats, barley, flax, and rye, and some wheat are cultivated. Chief rivers, the Foyle or Green river, the Bann or White river, the Faughan, and the Roe. The linen manufacture flourishes through every part of the county. Pop. 194,099."
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 Derry 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 Derry/Londonderry coverage includes Baptismal/Birth Records, Marriage Records, Burial/Death Records, Census Records, Gravestone Inscriptions, Griffith's Valuation (Free Access), and Census Substitutes.