A small depression, roughly 1.3 metres long and 0.8 metres wide, marks a point of collapse in the earth, and that subsidence hints at something underneath, a possible souterrain that has never been properly excavated or confirmed.

A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period in Ireland, often beneath or adjacent to a rath, the circular earthwork enclosure commonly known as a ringfort. They served various purposes, probably including food storage, refuge, and escape routes. The rath at Isertkelly is a known site in its own right, and this potential souterrain sits within its interior, centrally placed rather than tucked to the edge, which is a detail worth noting. Whether the collapse reflects a partially intact chamber below or simply a long-degraded void is a question the surface alone cannot answer.