Entirely submerged when researchers visited it in 2001, its only visible sign was a ragged cluster of rushes breaking the water's surface. Beneath those rushes lay a crannog, an artificial or partly artificial island of the kind built across Ireland and Scotland from the Bronze Age through to the early modern period, typically as a defensible dwelling place reached by a hidden causeway or boat.

The Corglass crannog is modest in scale, measuring roughly ten metres north to south and seven metres east to west, and it sits about a metre below the waterline while rising a metre above the muddy lake bed. Its edges are gradual rather than sheer, but the material beneath is noticeably different from the surrounding sediment, which is how it can be distinguished at all. The surface appears to be composed of small stones, and the landowner has described a more stable layer beneath those stones, consisting of brushwood supporting the stony cap above. A few mortised beams were also reported, the mortises being cut joints in timber, which suggests the structure was at some point framed with worked wood rather than simply heaped with material. That combination, stone over brushwood over jointed timber, is characteristic of how crannogs were built up over generations, sometimes across centuries of use and repair.