Donegal
Re Donegal Dublin surface transport time you'd be very lucky to complete the journey in 4 hours. The coach between The mid part of Donegal and Dunlin takes 5 hours. The airport is further north. I believe the airport (Carickfinn) is located about 45 minutes north of Dongeal town.
The airport supports the majority of Donegal county which is approx 160,000 people. But Donegal town is only 2,000 of that.