The feature in question is a souterrain, an underground stone-lined tunnel of the kind built throughout early medieval Ireland, typically for storage, refuge, or both. What makes the example at Coolnaha South quietly remarkable is how little of it can be verified from the surface, and yet how clearly something is there.

The cashel itself, a circular stone-walled enclosure of the type used as a farmstead or defended residence in early Christian Ireland, sits at Coolnaha South in County Mayo. Within its northwest quadrant, a slightly raised, sod-covered ridge runs across the ground, roughly two metres wide and only about thirty centimetres high. This low linear mound appears to follow the suspected course of the souterrain beneath, extending approximately twelve metres to the southwest from the cashel wall, then turning to the north-northeast for around eight metres, before turning again to the east-northeast for a further twelve to thirteen metres. The zig-zag plan is consistent with souterrain construction elsewhere in Ireland, where bends in the passage were sometimes deliberate, slowing intruders or perhaps directing airflow. Whether the surface ridge genuinely traces the buried tunnel below it remains unconfirmed; the evidence is suggestive rather than conclusive.