A series of hollows runs for roughly fourteen metres across the interior, and when the soil around them was probed, what came back was the resistance of drystone walling. The surface has been collapsing inward, which is precisely what happens when the roof of a souterrain, an underground stone-lined passage or chamber built during the early medieval period, begins to fail beneath the weight of centuries of accumulated earth.

The hollows sit close to the inner face of the bank on the north-west side of a rath, a type of circular earthwork enclosure commonly used as a farmstead in early medieval Ireland, typically defined by one or more earthen banks and ditches. Souterrains were frequently constructed within such enclosures and served various purposes, most likely storage and, in times of danger, refuge. They were built without mortar, relying on carefully fitted stone, which makes their survival, and their eventual collapse, a long and gradual process. At Tanrego, nothing has been excavated, and the passage below remains unconfirmed. What is known comes entirely from the surface: the hollows, the probing, and the geometry of a fourteen-metre line pointing south-east from the rath's north-western edge, suggesting something deliberate and constructed lies just out of reach beneath the field.