While there is much material for County Donegal on line and on microfilm, there’s a major repository for county records which should not be overlooked. This is the Donegal County Archives: http://www.donegalcoco.ie/services/communityculturalenterprise/ArchivesService/ It is part of the Donegal County Council.
Within its collections are a wide range of impressive record types, making it one of the largest county based collections in Ireland. Within its holding are the Poor Law Union archives which can be used to document the people in the workhouses. There are also Grand Jury records back to the mid-eighteenth century, school registers, estate records (landlord papers), private papers, court, legal deeds, Ordnance Survey maps and oral interviews. Out of all their collections, probably one of the most significant would certainly be the workhouse records from the local Poor Law Unions.
If you have Donegal ancestors the website inventorying the collections for the Donegal County Archives is worth consulting. They conduct research by mail, and the form isonline. You can also visit the archive by appointment. Information to arrange a visit is also on the website.
As any family historian with Donegal roots knows, the church registers tend to begin late. This reason alone makes the Donegal County Archives a major genealogical repository in the quest to document and extend your family lineage.