In reply to mountain_lion:
Regarding transport, trains in Ireland are still expensive, slow and infrequent. Every time I go back I rent a car from Dublin airport. There will be a lot of choice there. Get your excess covered from one of the online companies (e.g. insurance4carhire or icarhireinsurance). Driving to and from Dublin, especially if going south you will drive over a toll bridge, some car hire companies will take care of this , others don't. If they do not you need to pay in a shop or online fairly quickly before charges add up, I had to ring them up and beg ignorance etc and so they let me just pay the fee.
The roads have much improved, my town (Kilkenny) is 75 miles from Dublin, used to take 2.5 hrs, now just over 1 hr (if willing to go over the speed limit a bit, as you leave the Dublin area the roads empty out, sometimes driving home from the airport late in the evening I would be the only car in sight). Beware rush hour around Dublin though.
Regarding walks, scrambles, best to plan before hand as rights of way etc are not very common. Outdoor shops etc are also not very common so if you are not bringing every thing with you, look up shop locations beforehand.
Bring your swimming gear as most mountains are near the sea and there are lots of lovely empty beaches (water is a bit cold but not too bad).
Enjoy!