The entire case for its existence rests on a patch of ground that stays dry when everything around it is wet. A souterrain, for those unfamiliar with the term, is a man-made underground chamber or tunnel, typically constructed from drystone walling and roofing slabs, and commonly associated with Early Medieval ringforts across Ireland. They were used for storage, refuge, or both. The one at Liskillea leaves no visible trace on the surface whatsoever.

The dry patch was noted by Casey in 1983, and the reasoning is straightforward as archaeological inferences go: souterrains create air pockets and drainage conditions that prevent the kind of moisture retention you would expect in waterlogged ground. The absence of surface evidence does not rule one out, and in this case that anomalous dryness, persistent even in poor weather, was considered significant enough to be recorded formally. The souterrain sits, notionally at least, within the bounds of a ringfort, the circular earthwork enclosure that was the standard unit of rural settlement in Early Medieval Ireland, typically enclosing a farmstead and its outbuildings. Whether the passage beneath Liskillea was ever fully investigated, or whether it remains entirely undisturbed, the available record does not say.