The passage is a souterrain, a type of dry-stone underground chamber commonly associated with early medieval ringforts in Ireland, used variously for storage, refuge, or concealment. Whoever last moved that boulder intended, it seems, that nobody should go in. Or perhaps out.

The ringfort at Kilnabrack sits on low-lying ground overlooking Rossbehy Creek on the Iveragh Peninsula. It is D-shaped in plan, with an overall diameter of roughly 18 metres, and its enclosing bank reaches just over a metre in height. The bank, now largely reduced to a spread of collapsed stone, was recorded as an L-shaped feature on the second edition of the Ordnance Survey map, suggesting considerable deterioration over the intervening decades. The interior is overgrown and scattered with loose stone, which may represent accumulated field clearance rather than original fabric. The souterrain, listed separately as a distinct archaeological monument, appears at the midpoint of the southern wall, its entrance blocked by the boulder, and was marked simply as "Cave" on the same OS map, a casual label for something with a rather more deliberate history behind it.