Keem Bay is located near the western tip of Achill Island, one ridge and some very high cliffs away from the open Atlantic. The pocket beach is cut into the bottom of a small, steep valley. The bluffs behind the beach consist of rocky soil and glacial drift (I think) which shed all sizes of material, including enormous boulders, onto the beach. The stream that drains the valley cuts a small ravine behind the beach, then spills out across it.
Stones eroded from the bluff are reorganized by storm waves into a steep cobble berm. Heavier minerals are separated from lighter ones as waves wash across the upper beach, leaving thin layers of black sand. Finer sand is blown by the wind to form ripples and streaks across the beach.