A souterrain is a man-made underground structure, typically built during the early medieval period in Ireland, consisting of dry-stone walls topped with flat roofing stones called lintels. They were used variously for storage, refuge, or as escape routes connected to nearby settlements. The one at Lisaginny is, by now, nearly closed, its interior collapsed to heights of barely half a metre in places, which makes it less a feature of the landscape than a slow vanishing from it.

When archaeologist McCormick recorded the passage in 1978, it could still be described in some detail. The main stretch runs east to west, roughly 6.8 metres long and about 1.1 metres wide, entered through its original opening at the eastern end. At the western end the passage bends south, continuing for another 3.6 metres before the structure deteriorates further at its south-western corner. The whole thing sits on top of a gentle north-south ridge, positioned just to the south-west of the centre of a rath, which is a circular earthwork enclosure of the kind that served as a farmstead in early medieval Ireland. The pairing is typical; souterrains and raths frequently occur together across the Irish countryside, the underground passage functioning as an annexe to whatever life was conducted within the earthen banks above.