What draws the eye from the shore is not, however, quite what it appears to be. The low, dark mass of stone rising barely above the waterline is in fact a crannog, the term for an artificial or artificially modified island, typically built during the early medieval period as a dwelling place or place of refuge. This one sits in three to four metres of water, roughly seventy metres off the eastern shore, and for much of the winter it disappears beneath the lake surface entirely. In summer, when the water is clear, the submerged bulk of the cairn becomes visible below the surface even as the platform above remains slight and exposed.

The structure itself is a large, circular cairn of angular limestone blocks, measuring approximately fifteen metres north to south and fourteen metres east to west, with a level platform on top some eight metres across at its widest. The northeast face, the side that looks back towards the shore, is noticeably steeper than the other sides. There is no visible evidence of wooden posts, though these may survive beneath the waterline. What makes the site genuinely puzzling is its position. The water is too deep and the exposure to wind and wave action too severe for this to have functioned comfortably as a domestic settlement without significantly lower lake levels than exist today. Archaeologist Aidan O'Sullivan, who catalogued it as Faughalstown 1, has suggested it may instead have served as a boundary marker, a waypoint on a lake crossing, or an isolated location for safeguarding valuables or confining prisoners. The last possibility is given some weight by a possible iron slave-collar reportedly found on the island. Across the lake on the opposite shore lies a comparable early medieval crannog at Ballynakil, and it has been proposed that the two sites together may have marked a crossing-point of the lake. Nearby on land, a ringfort sits on a steep hill about four hundred metres to the northeast, and some six hundred metres to the east lies a rectangular earthwork known as Mortimer's Castle.