The structure at Killanully is a souterrain, a type of underground chamber or passage built in early medieval Ireland, typically associated with raths, the circular earthwork enclosures that served as enclosed farmsteads. This one curved through the earth for 11.3 metres, sitting nearly two metres below the present ground surface, and might have remained undisturbed indefinitely had the quarry not encroached.

When archaeologist Mount excavated the site in 1992, what emerged was a construction of two distinct techniques. The southern section of the passage was built using what excavators described as post and panel construction, a method involving upright timbers or stone posts with panels set between them. The northern end, closer to the entrance, shifted to drystone walling, with a handful of large upright stones, known as orthostats, placed near the opening. Only a single roof lintel survived in its original position, resting on a carefully built stack of drystone masonry. The rest of the roofing was gone. The passage itself had been deliberately backfilled at some point in the past, and the five distinct layers of fill that excavators worked through contained fragments of animal bone, mollusc shells, and charcoal, the quiet leavings of whatever activity once surrounded this place. Whether the souterrain served for storage, refuge, or some combination of both, as was common for such structures, the fill had long since sealed those questions in.