It is easy to overlook, and that is rather the point. The depression is thought to mark the collapsed remains of a souterrain, an underground passage or chamber built during the early medieval period, typically beneath or adjacent to a ringfort, and used variously for storage, refuge, or ventilation of living quarters above.

Raths, the circular earthen enclosures sometimes called ringforts, were a common form of rural settlement in early medieval Ireland, and it was not unusual for their inhabitants to construct souterrains beneath them. These underground features were built from stone-lined passages and corbelled chambers, and when their roofing eventually gave way, they left behind precisely the kind of surface depression visible here. The L-shape of the hollow at Masonbrook is suggestive: souterrains were frequently built with bends or turns in their layout, which may have served to slow unwanted intruders or to manage airflow. Whether the collapse here reveals a simple passage or something more complex beneath is not yet established, and the site is described as a possible souterrain rather than a confirmed one.