What looks from the shore like a natural feature is in fact a crannog, an artificial or heavily modified island constructed by human hands, of the kind built across early medieval Ireland as defended homesteads rising out of shallow water. This one, however, has shed most of the characteristics that usually define the type. There is no timber, no palisade, no obvious platform of brushwood and peat. What remains is a large subcircular cairn, roughly 22 metres north to south and 18 metres east to west, composed almost entirely of stone, with no earth binding it together, though kerbstones can still be made out along the eastern edge.

The lake itself is modest in scale, stretching around 475 metres along its longer axis and narrowing to between 100 and 170 metres across, with the eastern half falling into County Armagh. The crannog sits towards the broader northern end. Ordnance Survey maps from 1834 and again from 1907 record it simply as a large island, offering no suggestion that it was anything other than a natural formation. What erosion has done over time, though, is break the surface in places, and there the cairn gives something away: animal bones and charcoal have become visible, quiet evidence of occupation and perhaps the residue of hearth fires and domestic life at some unspecified point in the past. The absence of dateable material in the visible record means the period of use remains open, but the combination of structured stonework, kerbstones, bone, and charcoal speaks clearly enough to a human presence that was sustained rather than passing.