Local knowledge holds that it leads to a stone-built lintelled chamber beneath or within the wall itself, though no one in recent memory has been able to get inside to confirm exactly what lies beyond. That combination, a sealed threshold and an unverified interior, gives the place a particular quality.

A souterrain is an underground or semi-underground stone-built passage or chamber, typically associated with early medieval settlement in Ireland, and often interpreted as a place of refuge, storage, or concealment. The one at Reentrusk sits roughly six metres to the west of a church within a defined ecclesiastical enclosure, which suggests the broader site had a life as an organised early Christian foundation. Researchers O'Shea and Crowley noted as far back as 1972 that there were traces of a possible souterrain in the enclosing wall here, so awareness of the feature is not new. What has not changed since then is the fundamental uncertainty: the entrance remains blocked, the chamber unexamined, and its full extent unknown.