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It looks like you have found most options to select from a site, I would add a search with the name of the place and B&B, and that should show up all B&B's which do not like to join one of the bigger booking sites but still allow online or telephone bookings.
When I traveled Ireland I did so by train and bus, which did not allow for much flexibility in changing locations if you find everything booked solid.
With a car you are more flexible, you might find the nearest place with a room for you is half an hour driving away. Or you may find what a friend on a motorcycle with problems found, the only place he could reach was a posh (4 star?) manor house hotel.
So it is up to your risk avoidance or risk taking, I would not travel in summer without reservations but I do not even travel outside the summer without reservations. Ireland is quite popular and can at times be very busy with tourists, weekends round the year can already book out all B&B's in the more popular places, in summer I fear it can be impossible to find anything near the popular coast places and Dublin.
But I have not been there in summer.