We were in Ireland last summer in late August. I booked the October beforehand and was sure glad I did: our trip ended up coinciding with the Pope's visit and all of the sudden prices went up and options went out the window. My take on booking in Ireland: there seems to be plenty of housing options, but the later you book the more you pay.
We were super happy with every single one of our various lodgings, but for the hostel in Dublin. We should have splurged and simply stayed in a nice hotel upon arrival. We had a private room in a hostel, but the lack of a/c made it quite hard to sleep even in our jet lagged state.
Make sure to visit Newgrange and you need to get tickets ahead of time. Truly amazing to go right at 9 a.m. with a small early morning group.
We wish we had spent more time on the Dingle Peninsula. We actually stayed in a really amazing old home in Cloghane w/in walking distance of a beach and then we made the trip over Conor's Pass to visit the next day. We wish we had not bothered with the RS guide book recs that day, but simply followed the Dingle route around and stopped where the fairies beckoned.
We had pondered visiting Giant's Causeway, if on your list. At Newgrange, we discussed it at length with a couple who had made the trip. It sounded completely mobbed by tourists. They regretted going. Glad we did not spend the time going there.
The museum cafe food was amazing and we often enjoyed lunch and tea right at the local castle/museum/art gallery we were visiting. The local grocery stores are worth a visit too. We carried a food bag and were so glad we did. We were happy to pack our own picnics, and coming from a place that prides itself on organic and local, we were stunned by the quality and price of food in Ireland. I ate much lox, and I haven't eaten much since last summer!