Trip coming up 10/1-10/10. The itinerary is pretty firm on when we will be in Dublin and Belfast, but in the middle there are a few days where I'd like to be more flexible. We would be generally in the southwest and western parts of the country, not in the bigger cities. So, how necessary is it to book a B&B in advance, or will we be able to get a room the same day? These would be from Saturday to Tuesday nights.
Generally yes if flexible. Slightly better plan would be to call the day before but unless you hit a festival or a big weekend, you can find something. May not be your first choice.
in September 2013 we wanted to be flexible and we booked two days in advance in the town we planned on staying. we did not have a problem, we did not really have a preference on a B&B as long as it was rated good on Bookings.com or trip advisor. this saved us time from stopping at places to see if they had space when we arrived at a new location.
As others said, you can definitely find a room the same day if you are a bit flexible. I would go with a few ideas on where you might want to stay which can be helpful when calling to see if they have a room. Also, lots of the B&Bs will have signs with vacancy; if something looks good, just stop and check it out.
are you just a group of 2? when I was there in October w a group of 6 needing 3 rooms, we had a WICKED time finding accomodations.