It was. The island is a crannog, a type of artificial or partly artificial lake settlement built from stone, timber, and compacted material, used in Ireland from prehistoric times through the early medieval period and occasionally beyond. What makes this one quietly odd is its construction: not the usual timber platform over boggy ground, but a carefully assembled cairn of small limestone slabs and pebbles, shaped into a neat, flat-topped mound with sharply sloping sides and a continuous kerb of stone set around its perimeter. From the water, it rises about 1.45 metres above the present lake level, oval in plan, measuring roughly 31 metres east to west and 27 metres north to south.