A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts, and thought to have served for storage, refuge, or both. What makes the one at Knockrour particularly elusive is that it leaves no visible surface trace whatsoever. Its existence rests entirely on local tradition and the cautious observations of one researcher.

In 1939, archaeologist P. J. Hartnett recorded that there were indications, confirmed by local tradition, of a souterrain inside the rampart at the north-west of the ringfort at Knockrour. The ringfort itself is a separate recorded monument, and the souterrain is understood to lie within it, as was common practice. Early medieval ringforts, circular enclosures defined by earthen banks and ditches, were the farmsteads of Ireland's early Christian period, and souterrains were a fairly standard feature of the more substantial examples. Whether the Knockrour example survives intact underground, has collapsed, or was simply misremembered across generations of local knowledge, is not known. Hartnett's note is brief and careful, and no subsequent investigation appears to have resolved the question.