A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with nearby settlement; they were used variously for storage, refuge, or as escape routes. This one sits in the north-west quadrant of a ringfort, and the trees planted across the interior of that enclosure have done what centuries of slow burial alone might not quite have managed: they have erased every visible surface trace of both features.

The ringfort itself is a recorded site, and the souterrain was noted as lying within its bounds. Ringforts, the most numerous class of monument in the Irish landscape, were enclosed farmsteads of the early medieval period, typically defined by one or more earthen banks and ditches. The combination of ringfort and internal souterrain is well attested across Ireland, and in many cases the underground passage would have been accessible from within the enclosure, perhaps from a dwelling house. At Glenaglogh, that relationship between surface and subsurface is now entirely obscured, the conifer planting having levelled the visual evidence that would otherwise allow a visitor, or even a careful surveyor, to read the site from ground level.