Achill, an island off the W. coast of Ireland, in county Mayo, from which it is separated by Achill Sound. It is 16 miles long by 7 broad, and has a coast-line of 80 miles. Between it and a smaller island (Achillbeg) lies Achill Sound, a deep and safe haven. Achill Head, 2,222 ft. high, is the name of its S.W. promontory, that on the N. being called Saddle Head. The W. coast is steep, rocky, and dangerous: but on the E. the approaches are easy and sheltered, the sea being fordable at low water. The soil consists of rock and bog, and but little of it can be cultivated. Amethysts are found here, and there is a valuable bed of limestone.