It is the kind of feature that rewards those who know what to look for, because to a casual eye it reads simply as a depression in the earth.

A souterrain is a man-made underground structure, typically associated with early medieval ringforts in Ireland, and thought to have served as a place of refuge, storage, or concealment. The example at Castleblagh was investigated by Lee in 1932, who recorded two rectangular stone-built chambers arranged at right angles to one another, forming the characteristic L-shape still faintly legible in the landscape. The first chamber measured roughly 6.45 metres in length, 1.42 metres wide, and 1.47 metres high, roofed with flat stone lintels laid across the top. The second chamber had already collapsed by the time of Lee's investigation, leaving its dimensions unrecorded. McCarthy noted the site again in 1977, by which point the hollow in the southern interior of the ringfort had become the primary visible indicator of where the souterrain once ran.

The site sits within the enclosure of the ringfort itself, a circular earthwork that would originally have defined a farmstead or settlement of the early medieval period. The souterrain beneath it represents a considerable feat of dry-stone construction, cut into the ground and lined with upright slabs, the whole thing assembled without mortar. That one chamber survived intact long enough to be measured and described, while its neighbour had already given way, speaks to both the quality of the original build and the slow, uneven toll that centuries of pressure and moisture take on structures buried just below the surface.