The only places I know of are hotels. All the smaller venues will be closed Christmas day.
Here are some links in case you want to make further enquiries with the hotels. Christmas menus and prices are not available yet
Kees hotel Hotel Clanree and Gallaghers hotel can also do some Google search yourself to find more info. Answer 2
here is a website about christmas in donegal it covers places to eat at christmas. the website is about to be updated so this one is out of date at the moment. it covers christmas in donegal 2007. the 2008 version will be out this month. here is the link anyway. Answer 3
Sher Donegal is closed all year round......Answer 4
most of the big hotels should be open ,, go to discoverireland.com