Somewhere beneath the ground, in the vicinity of a rectangular enclosure containing standing stones and ogham stones, lies what is most probably the roofing of a souterrain, an underground passage or chamber typically built in early medieval Ireland for storage or refuge, constructed from large stone slabs set over a dug-out trench. There is no visible surface trace left today.

The evidence for it is slender but specific. The antiquary John Windele, whose observations were later cited by R.A.S. Macalister in 1945, noted during his survey of the site a solitary pillar with some flagstones laid close by, and read these as the remnants of a souterrain's roof structure. The flagstones, displaced or partially exposed, were apparently all that gave the feature away. The broader site at Ballynabortagh is already of considerable interest: a rectangular enclosure containing both standing stones and ogham stones, ogham being the early Irish script carved as a series of notches and strokes along the edge of a stone, most commonly associated with the early medieval period. The souterrain, if that is indeed what lies beneath those flagstones, would fit naturally into such a setting, as souterrains are frequently found in association with early ecclesiastical or settlement enclosures across Munster.