Dog's Bay is the site of a neat tombolo two miles south of Roundstone. The sandy isthmus has beaches on both sides and is wide enough to support dunes down the middle. We wandered out the beach on the western side (I think this side is technically the Dog's Bay side), but were chased back to the car by a squall.
We visited several beaches during our two days in Clifden, but this was the only one that merited posting. There were interesting beaches scattered along the shoreline near Cleggan, each quite different, but none that stood out and given the overcast weather, few that generated good photos. We looked across the sandy channel to Omey Island, but decided against driving across (although the low tide crossing was clearly marked). And I would have loved to explore the shoreline west of Ballyconneely, but it might have been tough to see through the downpour.
Beaches weren't the only thing to see in the area and there are a lot of pictures on my hshipman blog (e.g. Clifden, Connemara, Bog Road, Sky Road, and a couple of others).