The general advice is to book your first and last night. You will be tired on arrival and don't want to be searching for a place. You know where you need to be on your last night, so might as well book in advance.

For Kilkenny, I would book that portion. It is always busy there. Plus I am working with lots of clients going during your time period. Many have found things booked up already. 2013 is the year of The Gathering. It is already getting off to a hot start.

For the nights you "wing it" have a destination or two in mind and a list of B&Bs you are interested in. That may cut down on the time expended in searching for a place to stay.