This is a souterrain, a type of dry-stone underground structure built during the early medieval period in Ireland, typically used for storage or as a place of refuge. What makes the one at Cornaveigh quietly puzzling is not just its modest dimensions but the question it leaves open about its own layout.

The structure was discovered and investigated in 1985 by Power and Doody. It consists of a stone-built entrance chamber, roughly a metre long and 0.7 metres wide, oriented northeast to southwest, leading into a stone-built gallery set at right angles to it. The gallery runs five metres in length, just under a metre wide and 0.84 metres high, meaning anyone moving through it would have to crouch considerably. At the southwest end of the gallery, a lintelled entrance, stone slabs laid horizontally across upright supports to form a low doorway of 0.66 metres, opens into a further chamber that has since collapsed and is no longer accessible. That third space appears to have been earth-cut rather than stone-built, a simpler method of construction than the gallery preceding it. The investigators also raised the possibility that the entrance chamber itself may function as a second gallery, one that continues further to the northeast beyond what was examined. If so, the full extent of the souterrain remains unmapped. A fragment of iron slag found on the gallery floor hints at activity nearby, possibly metalworking in the wider settlement that would once have made use of this structure, though the precise context of that find was not established.