I highly recommend booking your B&B in advance. The vouchers we got through the tour company were valid in more than 2000 B&Bs across Ireland; however, in the more trafficed areas, like Dublin, I found it difficult to find an opening, and this was a month in advance.
I found it nice to know at least where we were staying, and then we could be flexible in what we saw around that area that day. We did meet up with a couple that were just "winging it" and with all the B&Bs that we saw I'm sure they managed to find rooms each day. However, I would prefer to use that time seeing the sights rather than going from B&B to find a vacancy. That's just my two cents of advice. :) In addition, we had requested ensuite rooms, with private baths. I believe, for less money, you can find accommodations with a shared bathroom, though I found ensuite to be more convenient.
Another tip -- depending on the B&B, some of them provide towels and washcloths, some provide just towels. Of the 6 nights we were there, two of the accomodations didn't have washcloths. I was advised to go to Wal-Mart and buy a bunch of washcloths for $5 and roll them up in my shoes and such in the suitcase to take them with me. I'm glad I was told that, because as I mentioned I needed them twice. Then, when you're done, just throw them out or leave them in the bathroom, so you don't have to take wet washcloths with you. They only cost you about 50 cents a piece, so no problem.
Here are some of the B&Bs we stayed in, with a mini review of each and a link to the full review.
1. Daly's House - Doolin, County Clare: This was our first B&B experience, and it was a cozy one. The house was a little hard to find - off the beaten path, but walking distance to the local pub with Irish music. Friendly family, good food, average size room ensuite with its own batheroom. On a scale of 1-10, Daly's was a 7. Read the full review entry.
2. St. Judes - Galway: This was our best B&B stay during our trip. The house was beautiful and very big, the room was large enough for two twin beds and one full, it had it's own bathroom with a towel warmer, a view out the window to a rose garden, and a heater (a lot of the B&Bs were on the chilly side.) St. Judes was a 9 out of 10. Read the full review entry.
3. Seefin - Dublin: It was really hard to find a B&B close to the City Centre in Dublin, so we stayed in the first one we found with availability. The rooms were small, and I believe there were only 3 rooms for guests in the whole house. If our beds were any closer together, we'd have been married. You couldn't even turn around to put your pants on. The hostess was nice enough, but not overly friendly. Seefin was a 4 out of 10. Read the full review entry.