Hi. I live in London, but my mother-in-law's home was in Donegal, and I've been an annual visitor there for 26 years now.
It's a beautiful area, but relatively remote and you need to remember that when travelling / staying. The largest town is Letterkenny - it has a number of hotels, and the largest (and most numerous) shops in Co. Donegal. It also has a number of pubs, bars and other entertainment facilities, including An Grianan Theatre. However it is to the east of the county, so to drive to the west or north coast of the county could take more than an hour. Ardara, Donegal town, Glenties or Killybegs are small towns (one main street, generally) and are conveniently situated to tour the far south/southwest of Donegal. All have cosy bars to enjoy. To tour the northwest of the county, best to stay somewhere like Dungloe, Gweedore or Dunfanaghy - large villages, really, with a few small shops/supermarkets, but all with good bars too. To visit the far north of Donegal (Inishown Peninsula) you would be best to stay in Buncrana or Letterkenny - unless you consider Derry / Londonderry, which is easily the largest city in the NW of the island of Ireland, with multiple shops, bars, hotels and pubs - it also has an airport and rail connection (to Belfast).
There are no 'world famous' single attractions in Donegal; rather, its interest lies in its scenic beauty; amazing coast and beaches, and the tiny bars you're sure to find. If you stay in the SW be sure to drive to see Slieve League, the highest sea-cliffs in Europe (close to Teelin). Also in the SW is Glencolmcille -also worth an hour. Donegal town is interesting (many pubs and also a genuine castle) and, about half an hour's drive from Donegal is Belleek Pottery centre (it's just over the border).
In central Donegal, next to beautiful Lough Finn, is Donegal's only working restored historic railway, the 'Fintown Railway'; in Dungloe is the Daniel O'Donnell Visitor Centre (a currently touring and well-established easy-listening singer, popular with a fanatic following!). Closer to Letterkenny is Glenveagh National Park - a stately home that was restored by Americans, plus thousands of acres of supreme mountain, lakes and red deer. It also has a sad place in the history of the Irish evictions of the 19th Century. There are also several worthwhile attractions in the NE of the county, but as it sounds like you want to concentrate on the SW, I won't detail them here.
I'm not a fisherman or animal watcher, but if you are, there are many ways of getting out to sea to fish, watch for whales, dolphins and seals, from most of the small ports dotted round the extensive coastline.
Wherever you stay, remember that although the mileage to drive may not look too bad, the roads are winding, indirect and unlit at night - it could easily take 3 hours to drive from the extreme NE of the county, to the extreme SW!
Have a great few days there.
