Hello
Donegal is beautiful, with a remoteness difficult to find in the British Isles other than perhaps in Scotland or parts of Wales.
I'm a Londoner, but my mother-in-law lived in Dungloe Co Donegal; I got married there, and have been travelling there for holidays for 25 years, so have explored most of the county - loads to do and see, provided that a) you're looking for a relaxed, mainly non-urban experience, and b) the weather stays fair for you.
Anyway, some considerations first, before places to see, things to do:
- You do indeed need a car. Public transport is almost non-existent in the county - no trains at all (with one tourist exception); an airport that only serves two locations directly - Dublin, and Glasgow; and buses and coaches that only run infrequently.
- By the standards of the British Isles, the county is large and the roads slow, so crossing the county in a car takes longer than you might expect. For instance, Ballyshannon (S) to Malin (N) is 82 miles and may take almost 3 hours; Lifford (E) to Dungloe (W) is about 52 miles but might take 90 minutes; Donegal Town (S) to Dunfanaghy (NW) is 52 miles and might take 90 minutes.
- Petrol (gas) stations are less frequent than they used to be, and opening hours can be erratic.
The City of Derry/Londonderry (a.k.a. 'Stroke City') is not in Donegal (or even in the Republic), but is the biggest, closest city if you need big stores. It also has a train to Belfast, and a (slightly) larger airport.
OK - so now the good news..
The scenery - stunning, almost everywhere. Beaches to die for; cliffs; bays; inlets; loughs; hills/mountains; peat bog/moorland; thousands of acres of nothing - not even cellphone signal! Bliss....Consider Slieve League, but also many (most) other places along the coast. Rossnowlagh beach is a famous surfing beach, but there are many other beautiful 'strands' - Portnoo and Narin; Cruit; Carrickfin (funnily enough, next to the airport! - but not an issue); Dunfanaghy; Portsalon; etc
Also consider driving the Donegal portion of the Wild Atlantic Way
Historical sights (my favourite): Dunree Fort; Donegal Castle; Doe Castle & the area around Creeslough; Fintown heritage railway; Donegal Town railway heritage museum (in a disused station); Flight of the Earls exhibition in Rathmullan; and others
Cultural Heritage: there's culture everywhere in Donegal, but some 'sites' might include Glencolumbkille; Daniel O'Donnell Visitor Centre; Newmills Corn and Flax Mill; Dunfanaghy workhouse; Dunlewey Centre (in the shadow of Mt Errigal the cone-shaped highest mountain in Donegal); Glenveagh National Park estate and castle; Doagh Famine Village (a bit kitsch, but still interesting - and in a beautiful spot). I could go on - and on.
I haven't mentioned the swimming, diving, walking, golfing either; or the music. If you have the time, visit the City of Derry, just over the border; and the various beautiful peninsulas - Fanad; Inishowen; Rosguill; etc
Plenty to see!