The lake itself, a small sub-triangular body of water nestled between drumlins in County Monaghan, has shrunk considerably over time, and the 1907 edition of the Ordnance Survey six-inch map still shows this feature surrounded by water. Now the lake has retreated, and what it has left behind is a cairn, roughly eighteen metres across and barely half a metre high, with trees growing from it and no obvious structural remains visible at the surface.

The site is classified as a crannog, a type of artificial or partly artificial island dwelling used in Ireland and Scotland from the Bronze Age through to the early modern period, sometimes built on existing shallows and sometimes constructed entirely from timber, stone, and compacted earth. What survives at Croaghan is more ambiguous. The mound is cairn-like rather than clearly domestic in form, and though no walls or post-holes are visible, the matrix of the mound contains charcoal and stone, which suggests human activity at some point, whether as a settlement platform, a burial feature, or something else entirely. The charcoal in particular hints at occupation or burning, though without excavation it is impossible to say more precisely what happened here or when.