"Donegal, a county of Ireland, in Ulster, 68 m. long and 44 broad; bounded W and N by the ocean, E by Londonderry and Tyrone, and S by Roscommon and the bay of Donegal, in a champaign country; containing 42 parishes, and sends 2 members to parliament; has extensive linen manufactures, and abounds with harbours. Pop. 249,483."
[From The New London Gazetteer (1826)]
Transcription of the 1630 Muster Roll can be found on Ancestry.
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 Donegal 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 Donegal coverage includes Baptismal/Birth Records, Marriage Records, Burial/Death Records, Census Records, Gravestone Inscriptions, Griffith's Valuation (Free Access), and Census Substitutes.