The site is believed to contain a souterrain, an artificial underground passage or chamber built during the early medieval period, typically constructed from stone and used for storage, refuge, or both. What makes this particular example quietly interesting is how little of it announces itself: the prostrate stone, embedded and horizontal, is all that currently points to what might lie below.

The souterrain sits within a rath, the Irish term for a ringfort, a type of enclosed farmstead defined by one or more earthen banks and ditches. Raths are among the most common archaeological monuments in Ireland, built predominantly between roughly the sixth and tenth centuries, and it was not unusual for their builders to incorporate souterrains into the design. The underground element would have been accessible from within the enclosure, often concealed and approached through a narrow entrance. At Carrigeencullia, the rath itself is a recorded monument, and the souterrain associated with it carries the status of a possible rather than confirmed feature, with the embedded prostrate stone in the rath's centre offering the strongest physical suggestion of its presence.