The disturbance exposed a souterrain, an underground stone-lined tunnel of the kind built across Ireland during the early medieval period, typically associated with nearby settlements and used variously for storage, refuge, or both. This one had been sitting quietly beneath the soil, unrecorded and undisturbed, until the machinery broke through.

The passage was constructed in drystone, meaning the walls were built without mortar, and ran roughly north to south for approximately 6.5 metres. It was not uniform along its length; the tunnel widened as it extended northward, ending in a rounded terminus. A small recess was cut into the northern end of the east wall, the kind of feature sometimes interpreted as a niche for a lamp or a small storage hollow. Entry after the discovery was made through the accidental breach at the southern end. The structure was subsequently sealed and is no longer accessible.