Cairns have been constructed in Ireland since the Neolithic, when they covered megalithic tombs and were in use until the fourth or fifth centuries A.D. Unlike barrows cairns are a by-product of agricultural clearance and in upland areas and on thin soils covering exposed geological formations would have been a readily available building material. Cairns are usually of three types. High cairns, resembling bowl barrows, which often covered passage tombs, much lower cairns of less than 2m in height with flat tops and ring cairns, which enclosed a central burial. A number of the cairns covering megalithic tombs had Bronze Age cists added to them or had the central chambers re-used for Bronze Age burial. Knocknarea Cairn Co. Sligo