A shallow circular depression, roughly two metres across and less than half a metre deep, sits within the earthwork, with a single large stone set firmly into its floor. The most likely explanation is that this marks the roof of a collapsed souterrain chamber.

A souterrain is an underground stone-lined passage or chamber, typically associated with Early Medieval ringforts and raths across Ireland. They were built between roughly the sixth and twelfth centuries and served various purposes, most plausibly as cool storage for food, as refuges during raids, or as escape routes. The rath at Keelties is itself a ringfort, the remains of a circular earthen enclosure that would once have defined a farmstead or small settlement. The depression, measuring approximately two metres on its longer axis, sits directly within this enclosure, suggesting the souterrain was integral to the original construction rather than a later addition. The firmly set central stone may be a capstone or lintel from the chamber below, left in place as the surrounding fill collapsed around it over centuries.