It leads into a stone-built underground chamber, a souterrain, that was carefully constructed with drystone walls of horizontally laid rough slabs, corbelled inward at the top to support a single large roof stone. The chamber measures roughly 1.9 metres by 1.35 metres, with a standing height of 1.3 metres. Modest dimensions, certainly, but the structure's real strangeness lies in what it was reportedly used for, long after whatever original purpose it served had been forgotten.

Souterrains are underground passages or chambers built, typically in the early medieval period, most likely for storage or refuge. The one at Cloonmore sits just twelve metres west of a ruined church and its associated graveyard on the ridge above. When Ordnance Survey officers were gathering local knowledge in 1838, they recorded an oral tradition attached to a cave said to lie to the south of that church. According to what they were told, a schoolmaster once taught inside it during the period when Catholic education was suppressed under the Penal Laws, and a priest celebrated Mass there for a congregation when the practice of the faith carried serious legal risk. Those laws were enacted progressively from the late seventeenth century and remained substantially in force through much of the eighteenth. Whether the cave described in 1838 and the surviving chamber to the west are the same feature, or once connected, is unclear. No continuation of the passage is visible to the east or south, and on the western side the structure appears to have been cut short by the road that now runs alongside it, though remnants may still lie beneath the tarmac.

The chamber today is partly filled with loose rubble and earth, which obscures the floor and the lower courses of the walls. The opening itself sits at the foot of a field fence, facing west onto the road. It is the kind of place that most people would pass without a second glance, which is perhaps fitting for somewhere that once depended on not being noticed.