That hollow is, in fact, all that remains of a souterrain, an underground stone-lined passage or chamber typically built during the early medieval period, often for storage or as a place of refuge. The structure has long since collapsed, leaving only the ground-level ghost of what lay beneath.

The site was noted in 1945 by the archaeologist M. J. O'Kelly, who described the feature as an elongated hollow marking the position of the fallen souterrain. O'Kelly went further, suggesting that an ogham stone, one of those remarkable early medieval markers inscribed with a script of notched lines representing letters, had been reused as a roofing slab within the souterrain before its collapse. That detail is intriguing, since repurposing an ogham stone as raw building material would imply the inscription had already lost its significance to whoever constructed or modified the passage. However, the ogham stone in question had first been described in 1897 by T. Barry, and his account does not support O'Kelly's claim about its origins. The two sources simply do not agree, and the question of where exactly the stone was found, and how, remains unresolved.

The hollow O'Kelly recorded is reportedly still visible within the enclosure, which gives the site an oddly legible quality despite the absence of any standing remains. Knowing what to look for, a subtle elongated dip in the hilltop ground, turns a featureless landscape detail into something with a rather complicated history attached to it.