That claim alone would make this place worth pausing over, but the souterrain, an underground stone-lined passage or chamber typically associated with early medieval ringforts and used for storage or refuge, has not given up its secrets easily. What survives above ground is mostly suggestion: a U-shaped depression in the southern half of the ringfort, a circular hollow a few metres to the north, and a partially embedded stone slab near the western bank that may once have formed part of the roof.

The fullest account comes from Broker, writing in 1937, who recorded that men had ventured in a few hundred yards and found what they described as an underground chamber with fine rooms roofed with flagstones. The supposed connection to Kilmeady Castle is the kind of detail that accumulates around souterrains across Ireland, where folklore frequently extends their reach far beyond what archaeology can confirm. Whether or not any physical passage ever ran that far, the tradition points to how seriously people once took these underground spaces, associating them with places of power and significance in the local landscape. Local information suggests the souterrain was still accessible in relatively recent times, which raises the possibility that the fabric of those flagged chambers may be better preserved than the surface depressions alone would suggest.