That ambiguity is itself the story. A souterrain, for those unfamiliar with the term, is a man-made underground structure, typically stone-lined, associated with early medieval ringforts and used variously for storage, refuge, or ventilation of perishable goods. They are found across Ireland in considerable numbers, and many were built in close association with a rath, the circular earthen enclosure that served as a farmstead during the early medieval period. The rath at Knocknahila is a documented site. The souterrain beside it, however, exists only in local memory.

The published record for the Knocknahila rath makes no mention of any souterrain in its vicinity. The sole basis for believing one might be there is described as local tradition, a phrase that covers a broad range of possibilities, from a half-remembered family story to an oral account passed down with some consistency over generations. Local tradition is not nothing. Many genuine archaeological features were first flagged to researchers through exactly this kind of community knowledge, and some have later been verified through survey or excavation. Others have not. In this case, no further investigation appears to have resolved the question one way or the other, leaving the Knocknahila souterrain in a peculiar state of suspended uncertainty, documented as a possibility without being confirmed as a fact.