The dark, rounded stones are primarily limestone and shale, originating from the surrounding Burren Hills. These have been dislodged by millennia of erosion and glacial activity and smoothed by the sea.
This stone beach is a textbook example of a 'storm beach'. During severe winter storms, the ocean crashes against the coast with tremendous force. The lighter sand is then often sucked away into deeper water, exposing the heavy pebbles (which the current cannot move). In calmer summer months, the sea often deposits a layer of sand over these stones again.