A souterrain is an underground passage or chamber, typically built during the early medieval period in Ireland, often associated with storage or refuge and usually constructed from stone-lined walls with a capstone roof. The trenches at Lisnalurg curved inward toward the interior bank of the enclosure, which itself had sustained a large breach at the point where they terminated. Whether they represent a genuine souterrain, a collapsed one, or something else entirely was left, even at the time, as an open question.

The 1973 report is the only moment these features appear clearly in the record. Subsequent field inspections found no trace of them. It is not clear whether they had been obscured by vegetation, backfilled, or simply missed; their absence from later accounts gives the original observation a slightly ghostly quality. Features like this can vanish quickly in agricultural land, particularly if a breach in a bank has invited further disturbance over the intervening decades. What was noted with some precision in 1973 had, by the time anyone looked again, effectively ceased to exist as a visible feature on the ground.