That ambiguity is itself the point. A souterrain is an underground stone-lined passage, typically associated with early medieval ringforts, used variously for storage, refuge, or ventilation of living quarters. The one at Tawin has left no trace on the surface and, when surveyed, none underground either.

What prompted anyone to look in the first place was a reference in Holt's 1912 account, which described a second mound lying to the west of the visible mound of earth and stone inside the rath. Holt noted that this secondary mound apparently concealed the entrance to a souterrain. By the time surveyors came to examine the site, that mound had vanished along with any surface indication of what lay beneath it, and no evidence of the souterrain itself could be found. Whether it was destroyed, robbed of its stone, or simply never extended far enough to leave a detectable trace is unknown. The rath it belongs to, a roughly circular enclosure of the kind built across Ireland during the early medieval period, survives nearby, making the missing souterrain all the more tantalising as a companion feature that seems to have slipped entirely out of the record.