The entrance has long since been filled in, and no surface trace survives today, yet the tradition of its existence has persisted in local memory with remarkable consistency.

A souterrain is a man-made underground tunnel or chamber, typically associated with early medieval ringforts, where it may have served for storage, refuge, or escape. The passage at Ballinlass was linked to a local ringfort, a circular earthen enclosure of the kind that dots the Irish landscape in the thousands. As early as 1914, a published account recorded the local tradition that the souterrain ran from this ringfort, with its far opening located on the southern uplands some distance away. Local people once knew how to enter it through a gap in the northern part of the garth, the enclosed yard or courtyard area associated with the fort, and it was understood to exit near a second earthwork to the west. The two monuments are thus connected not only by the supposed underground passage but by a shared folklore, with local knowledge threading between the two sites long after any physical access became impossible.