A shallow depression near the inner edge of a rath, a circular earthwork enclosure typical of early medieval Ireland, is the only visible hint that something lies below. That gentle hollow in the northwest quadrant of the rath is thought to mark the entrance to a souterrain, an underground chamber or tunnel built during the early medieval period, most likely between the seventh and twelfth centuries. Souterrains were constructed from stone and earth, used variously for storage, refuge, or ventilation, and are found across Ireland in their hundreds, often associated with exactly this kind of enclosed farmstead site.

The rath itself, recorded separately, forms the immediate context for the souterrain. A personal communication from M. Keane in 1991 placed a souterrain within its interior, noting that the slight topographic anomaly close to the scarp, the sloped inner face of the earthen bank, might indicate where it sits. Beyond that single observation, the underground structure has not been excavated or formally documented in any detail. What lies beneath the depression, whether a simple corbelled chamber or something more elaborate, remains unconfirmed. The site at Killacorraun is one of many such quietly ambiguous places scattered across the Mayo landscape, where the evidence is suggestive rather than conclusive, and the ground holds more than it reveals.