The Donegal County Museum is on the High Road in a stone building completed in 1843 as the warden's house of the Letterkenny Workhouse. It opened as a museum in 1987 and covers local history from the Stone Age through the Plantation of Ulster and beyond.
Entry is free, and it is a good place to get your bearings before heading out into the county. Check opening hours before you go; it has been open weekdays and Saturday afternoons.