The opening is choked up, the ground above it gives nothing away, and the whole thing would be easy to walk over without the faintest suspicion that anything lay underfoot.

A souterrain is a man-made underground passage or chamber, typically dry-stone lined, associated with early medieval ringforts across Ireland. They were used variously for storage, refuge, or ventilation, and their builders were evidently skilled at integrating them into the fabric of a defended enclosure. The one at Lismahane sits concentrically inside the inner rampart to the south, a detail recorded by P. J. Hartnett in 1939, who described what appeared to be an extended underground passage but noted that its entrance was already completely blocked. That brief observation is essentially all that is known. No excavation appears to have followed, no later survey has reopened the question, and the surface today shows no trace of what lies beneath. The ringfort itself remains the visible feature; its hidden interior is, for now, inaccessible in the most literal sense.