A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlement in Ireland, and usually found within or adjacent to a ringfort. The one at Lismeelcunnin is notable less for what it is than for how little of it remains.

By the time a researcher named Bowman documented the site in 1934, the souterrain was already described as almost destroyed, sitting on the fosse side of the south-west rampart of the enclosing ringfort. A fosse is the external ditch dug to create the raised earthen bank, or rampart, that defines a ringfort's boundary, and it is along this outer edge that the underground feature was recorded. Since Bowman's note, the site does not appear to have improved. When archaeologists later visited to inspect it, the fosse and bank were found to be heavily overgrown and effectively inaccessible. Whatever structural evidence once remained of the souterrain has likely continued its slow return into the ground.