Thats a good crossing to do the north and Donegal, everything will be open from then onwards, 2 weeks is the min time to spend, but id say go for 4 weeks at least.
Also a lot depends on what you thing is, some like pubs others history as in old buildings/castles etc, then there is the beach bums and not to forget shops.
Give me a shout before you land and what is your thing and I may be able to give a few pointers and a route, one thing is don't get obsessed with the wild Atlantic hype as you will miss a lot on the way around.