I agree with everything that Ranger said.
The NW is the prettiest area I was in, it has old country feel, and the people are the hardest to understand when speaking. In other words many of them still speak Irish.
When I go to Ireland I never venture out of Donegal anymore. Unless I go to Dublin.
I loved Galway and the Aran Islands. And if there is a meeting you really need to go to a horse race in Clonmel. Waterford is nice but it is a long way over there. Most of the south is very touristy. In Donegal you may not see another tourist for a couple of days.
I have been there a few times during mid-late June and never have had a problem finding a B&B. If you stop by one and they are full, they will find you accomodations. It happened every time.
I would suggest public transportation from one area to another and then just rent a car where you end up. Traveling by train is fun in Ireland because you can get off when you want to and get back on. The buses are frequent between cities and areas and much cheaper than car rental and gas.
When you get to Donegal the only place to rent a car will be Letterkenny. It is the County Seat and pretty centrally located. You will definitely need a rental car in Donegal. It is a rural area and there are not many buses in comparison to the other parts of Ireland.
If you have any specific questions please feel free to e mail me at wmka89 at y --oo. I will gladly help with any plans.
The first time I went we flew into Dublin, stayed a couple of days and then caught the first train out of Dublin. The first train was going to Ballina, so that is where we went. It was a great way to travel.