What makes this site quietly remarkable is not merely its age or its construction, but one particular detail at the far end of the second chamber: a stone seat, forty centimetres high, running the entire length of the back wall. Someone, at some point, built a place to sit.

The structure is a souterrain, an underground passage and chamber system of the kind built across Ireland, most commonly during the early medieval period. The word comes from the French for "underground passage", and these constructions served various purposes, most likely storage, refuge, or both. The Ellistronbeg example is a well-made one. The entrance, partly blocked by boulders, leads into the first chamber, which measures roughly four metres long and just under two metres wide, its roof formed by four large stone slabs sitting on corbelling, a technique in which stones are laid with each course projecting slightly inward to bear the weight above. From the southern wall, a low creepway, only half a metre high and two and a half metres long, connects to the second chamber, which is considerably larger at nine metres in length and oriented north to south. Ten capstones cover it. The overall construction, two distinct chambers at different orientations, joined by a narrow crawl-through passage, is a layout found at some of the more elaborate souterrains elsewhere in Ireland, suggesting this was built with care and likely for sustained use.