That scale is easy to miss now. The visible hollow is only around 1.4 metres wide and a metre deep, and much of what was there has either collapsed inward or been obscured over centuries. But the alignment is still discernible, running north to south, and the basic form of the thing persists even in its degraded state.

Souterrains are stone-lined underground passages or chambers, typically associated with early medieval settlement in Ireland, roughly between the seventh and twelfth centuries. They were built beneath or close to homesteads and enclosed settlements, and their exact purpose is still debated, though storage of perishables, refuge during raids, and simple concealment of valuables are among the most plausible explanations. This one sits near the centre of a cashel, which is a stone-walled enclosure of the same general period, a kind of fortified farmstead built without mortar. The combination of cashel and souterrain is not unusual in the Irish midlands and west, but finding both on the same site in any condition is always a small piece of good fortune. Here in Rannagh, the souterrain has clearly suffered significantly, its stone fill suggesting either deliberate backfilling at some point or the gradual slumping of its roof and walls. What remains is fragmentary, but it is enough to indicate the original scale and intention of the structure.