The suspected feature is a souterrain, a type of dry-stone tunnel built during the early medieval period, typically beneath or beside a rath, the circular earthen enclosure that served as a defended farmstead. Souterrains were used variously for storage, refuge, or concealment, and they are common enough across Ireland, though most are only discovered by accident, when a farmer's machinery breaks through a roof-slab or livestock stumble into a collapsed section. At Treankeel, the situation is more tentative still: there is no visible trace at ground level.

What keeps the possibility alive is local tradition. The story holds that a cave or underground passage connects this rath to a second rath located roughly five hundred metres to the north. Linking tunnels of that kind are occasionally reported in Irish folklore, and while they are rarely confirmed by excavation, the tradition itself is worth taking seriously as a pointer toward something genuinely buried. A passage of that length would be unusually ambitious by the standards of known souterrains, which makes the story harder to verify but no less interesting to consider. For now, the site sits quietly inside its earthwork enclosure, the ground giving nothing away.