The Yellow Pepper restaurant in Letterkenny is lovely. Its just across the road from the Lemon Tree and you won't have far to travel to any of the many lively 'watering holes' after your meal. The number is:
Tel: (074)9124133
Another suggestion is the Orchard Bar & Restaurant, which has a lovely restaurant and also has a few different bars and a disco upstairs as well, so you wouldn't have to travel anywhere i.e. an all in one location & I think they give concessions for the nightclub and champagne to hen parties, but you'd have to confirm this with them. Sorry I haven't got a number for them.
Some good pubs include:
The Vodo bar, The Brewery, The Cavern
Hope you get sorted. :)