This is a souterrain, an underground stone-lined chamber of a type constructed throughout early medieval Ireland, typically for storage, refuge, or both. What makes this one quietly interesting is its situation within a conjoined ringfort, a double enclosure in which two circular earthwork enclosures share a common boundary, and the souterrain occupies the western half of the northern rath, that is, the northern of the two earthwork enclosures.

The chamber itself runs north to south and measures 8.6 metres in length, built using the drystone technique in which carefully selected stones are stacked and fitted together without any binding mortar, relying on weight and geometry for stability. At its southern end, a trapezoidal depression extends 8 metres to the east, suggesting either a second passage or a collapsed adjunct space whose original purpose is no longer easy to read from the surface. Access, as it survives today, is at the north-west corner of the chamber. Souterrains of this kind are generally associated with ringfort settlements of the early medieval period, roughly the sixth to twelfth centuries, and were often used to keep dairy produce cool or to provide a concealed retreat during times of threat. This example, recorded as part of the wider ringfort complex at Ballyglass, is a small but precise piece of that pattern.