I think Petunia's idea of touring the north coast would be fab! Especially if Donegal is really high on your list of things to see. If you don't have a lot of time, I would fly into Belfast and rent a car from there. Just getting to the ferry in England can be a long journey in itself. Personally, I would skip Dublin on this trip and worry about that one when you decide to go south to Blarney.
fyi - my friend occasionally drives from Cork to Donegal and it takes her +/- 8 hours depending on traffic, bed pushes, rain, etc.