We've stayed at Fitzpatrick Castle, and rented a car while we were there. We took the train into Dublin--that was easy--and we drove elsewhere; driving elsewhere, however, wasn't easy. There are all the roadway problems others have mentioned, and there is the additional problem of needing to double the time to drive to anyplace from what you estimate would be requirred in looking at the map.
On the train coming back from Dublin, locals asked us what we were going to do the next day. We said, drive to Waterford, tour the factory, check out the Blarney Stone, and drive to the Rock of Cashel and back to Dalkey. Their eyes kept widening, and finally, one said "It can't be done." Another said, "Really, it simply can't be done." On the map it looks perfectly reasonable. We thought they just didn't understand what skilled driver and navigator they were looking at. Of course, they were right: "It simply can't be done."
We got where we were going and saw lots--but not "in U.S. time."
Another trip, we stayed on the west coast in a bed and breakfast--and rented a car. Our landlady said, "You'd be better off to take a local tour with a friend of mine to see the circle; it doesn't cost much, and you'll see a great deal more." This time, we believed, and took the drive in an 8 passenger van with her friend. We were glad we did.
In short, if you're not wanting to go to too many places that are really "out of the way," I think you'll do well with public transportation. We've done three trips with cars now, and if we go again, we'll do public transportation and skip the car.