Around Donegal Town there are a variety of place, some I've tried, and some I want to try!
Aroma @ The Donegal Craft Village. Very small cafe, nice entrees and great deserts especially the Orange Cake, and decent coffee. Inexpensive. Say Hello to Jackie for me. ;)
Blueberry on Bridge Street: Very good basic lunchs, great soups. Inexpensive.
Also, on Bridge St, a very surprisingly good Italian place.
Coxtowns Manor, near Ballintra: Old country Manor now owed by a Belgian. Food is supposed to be very good, all organic and much of it locally sourced. Place has a lot of charm & history. (Haven't tried it yet, but this summer I will).
Harvey's Point @ Lough Eske. Belgian owned, french influenced cooking. Very good, Expensive, great views of the lake & mountains, but building a bit modern looking and out of place for such a senic spot.
St. Ernans (near the Donegal Craft Village). I haven't tried this, it's very expensive, food is supposed to be really good. It's on a island connected to the shore via a causeway. Also, supposed to be a romantic place to stay.
Castle Murray: Between Donegal Town & Killybegs, on St. John's Point. One of the nicest meals I've had in Donegal, & good service.. Price Mid-range. Nice views of Donegal Bay. Call ahead to book. Also, a high class B&B.
Burtonport, in the NW of the county: Skippers Tavern & The Lobster Pot are 2 good seafood place, maybe the best I've tried in Donegal. Skipper's is very informal, but I think some of the best seafood I'd tried in a long time. The chef is spot on, place feels more like a pub with grub that a restuarant, which show in the service & ambiance a bit, so it's not a romantic spot, but I really think the food is worth checking out (or it was 2 years ago). The Lobster Pot is a lot nice looking, and the owner is from the US, but I don't think the food is as good.
If your in Letterkenny, there are 2 decent places down the bottom of main street, the Lemon Pepper and another place directly across the street (don't remember the name) that have been recommended to me.
Thanks for the list which is much more than I have found in any guide book. Are you from Donegal Town?