The shape is the collapsed remains of a souterrain, an underground passage or chamber, typically constructed during the early medieval period as a place of refuge, storage, or concealment. What makes this one quietly arresting is precisely its indistinctness: it cannot be picked out from aerial photography, and yet, walking the interior of the ringfort it once served, the earth itself maps out its former course.

The souterrain lies within a ringfort, the circular enclosed settlements that once dotted the Irish countryside in their thousands and remain one of the most common archaeological monument types on the island. The depression here follows a curvilinear path. It is broadest and deepest, reaching roughly one to two metres deep and two metres wide, at the point where it meets the inner bank at the south-south-east. That widened area is thought to represent the collapse of an underground chamber. From there, a narrower trench, about a metre wide and half a metre deep, extends approximately eight metres on an east-west axis. The passage then curves and runs a further ten metres north to south, growing gradually shallower and broader before fading out entirely at the northern end. The overall form suggests a passage with at least one chamber, now open to the sky only because the roof has fallen in.