We were in Ireland last year during the same time frame as you will be this year. We did get the B&B's we wanted but not always our 1st choice. We made our reservations prior to our trip. July is a popular month to visit Ireland. On our first visit to Ireland we had several nights that we hadn't made a reservation. I think we spent too much time trying to find a B&B for those nights.
Buncrana is in County Donegal, right? Since you will be in County Donegal you have some choices as you head back toward Shannon. My suggestion would be to drive toward the Connemara and drive the Mayo Loop unless you want to explore County Sligo or parts of Northern Ireland. I am not familiar with either of these two places. Rick's driving tour of the Mayo Loop begins at Galway but you could start at Castlebar heading toward Westport and pick up the loop that way. You'd see some lovely scenery, bogs, lakes, mountains, Kylemore Abbey and Connemara National Park just to name a few places as you circle the area. You might pick a place to stay along the loop for one or two nights. Cliften is a good choice. You could also drive toward Galway from Donegal with a stop at Cong if you prefer not to drive the whole loop. Next, I'd explore the Burren with a stop at the Cliffs of Moher. Two nights near the Burren would be great. Finally, you could spend the night before your flight at a B&B near Bunratty Castle and Folk Park. Shannon airport is about 10 minutes from Bunratty.
Barbara