It looks, at first glance, like a natural feature of the lake, the kind of reed-fringed islet that dots the drumlin country of the Irish midlands. In all likelihood, however, it is nothing of the sort.

The island is recorded as a crannog, an artificial or partially artificial lake dwelling constructed by piling timber, stone, peat, and brushwood into shallow water to create a stable platform for habitation. Crannogs were built and used in Ireland across a remarkably long span, from the Neolithic period through to as late as the seventeenth century, and they served various purposes depending on the era: family homesteads, refuges in times of conflict, bases for local lords. The modest diameter of the Annagharnet example, around the size of a large farmyard, is fairly typical of the form. Whether it was built up from the lough bed or constructed around a natural shoal is not recorded, but the apparently circular outline, still visible as a distinct island, suggests the underlying structure has held its shape across the centuries.