The quarrying exposed a construction trench in the quarry face, and what lay inside it turned out to be a souterrain, an underground passage built from dry-stone walling and covered with flat flag stones, of the kind constructed throughout early medieval Ireland as places of refuge, food storage, or concealment. What makes this one quietly remarkable is its scale and complexity: not a single corridor but five distinct chambers, arranged in sequence and varying considerably in their proportions.

The dimensions recorded at the time of discovery give a clear sense of how the structure was designed. The chambers range from a cramped passage just half a metre high and two metres long to a more generous space six and a half metres in length with a ceiling reaching just over a metre at its tallest point. Most of the chambers would require a person to crouch or crawl, which is entirely typical of the form. Souterrains were not meant for comfort; they were functional, and their low profiles helped retain cool temperatures and made armed entry difficult. The site sits in a field known locally as "Forteen", a placename that may preserve a memory of a ringfort, the circular enclosure, usually defined by an earthen bank and ditch, within which early Irish farming families lived. Souterrains are very commonly found in association with ringforts, built beneath or beside them, and the Dunbeacon example almost certainly belonged to just such a settlement, even if the surface features of that enclosure have since been lost to agriculture or time.