Ireland house rental

Without knowing more detail about your proposed itinerary (length of stay, specific areas of interest, etc), would suggest Homeaway.com and its sister site VRBO.com for self-catering accommodation. We had good luck with http://www.cottages4you.co.uk/ last year when we did this ourselves for several weeks. Some truly unique properties on offer (we found a couple of cute thatched cottages), and their filters will allow you to customize your search according to your specific criteria.
We find that self-catering saves a lot of money versus hotels or B&B's.
One of your issues will be a car - one that's large enough to accommodate 5 people plus luggage and still be maneuverable on some of the narrow country lanes you'll experience if you go much off the beaten path. If you'll be staying primarily in the cities you won't need, or want the hassle, of a vehicle, but for touring the countryside we found a having a car much preferable to tour buses.
There's a whole other discussion about driving in Ireland that I'm sure others will chime in on, but we enjoyed the freedom that having a vehicle provided, although driving on the left (with manual transmission) did take a little getting used to.

We rented a house in Killarney for two families in the summer of 2014 via VRBO. We stayed 5 nights and did the gap of Dunloe, a tour of Muckross House and Farm, the ring of Kerry (with a guide) and a day trip to Kenmare from this location. We visited Dingle the day we left Killarney, and then drove on to Doolin where we stayed 2 nights before heading to Dublin. We rented a nine-seater van that held the whole group, and didn't find it too big to maneuver, even when the GPS took us through some pretty marginal roads! We did assign one adult to navigate/remind the driver to stay left whenever we were in the car. I was the main driver, and did find the long drives tiring, so might have preferred to shorter stays at more places, though it was really nice to have the house to relax in on our downtime.