The shape is deliberate, not a trick of erosion or subsidence, and it traces the collapsed roof of a souterrain, an artificial underground passage built during the early medieval period, typically from around the seventh to the twelfth century. Souterrains were constructed using large stones or timber lintels, and served a variety of purposes including storage, refuge, and possibly ventilation for the ringforts, or circular enclosed settlements, within which they were almost always built.

This particular souterrain sits in the northern half of a ringfort, a circular earthwork enclosure of the kind that once numbered in the tens of thousands across Ireland and served as farmsteads for early medieval families. What remains visible at ground level is a hollow running roughly ten and a half metres in total. The main arm, stretching east to west, measures five metres in length and nearly two metres wide. About halfway along it, a second passage branches off to the north, running five and a half metres before terminating just inside the bank of the ringfort itself. Together they form the characteristic T-plan that gives this souterrain its quietly geometric presence in the landscape, a shape that would have been invisible to anyone standing above it when it was in use. The junction between the two arms is where a visitor might have once crouched to choose a direction, feeling the cool of the earth on either side.