During construction work on the track, the ground gave way to reveal an earth-cut souterrain, the kind of subterranean passage-and-chamber system built in Ireland roughly between the seventh and twelfth centuries, most likely for storage or refuge. What makes the Lissacresig example quietly remarkable is its complexity: not a simple tunnel or single room, but a sequence of four chambers connected by narrow creepways, the low constricted passages linking one space to the next, which would have forced any intruder to enter slowly and vulnerably, one at a time.

The structure sits on a gentle rise at a break in a south-facing slope in County Cork, with outcropping rock visible on the higher ground to the north and west. The entrance chamber, roughly oval and orientated east to west, now has its roof opening blocked with stones and cannot be accessed or measured. From there, a creepway just 0.56 metres wide and 0.34 metres high leads into Chamber 1, a near-rectangular space of about 3.1 metres by 1.75 metres, with flat roof, straight walls inclining inward toward the top, and a recessed construction shaft visible at the side. Construction shafts are vertical openings used during building to remove spoil from the digging; their presence here tells something about how methodically the whole system was engineered. Chamber 2, the largest, is pear-shaped, roughly 4.2 metres north to south, but its roof has partially collapsed at the northern end, and fallen rock litters the floor. A third creepway, only 0.4 metres high, connects westward into Chamber 3, another rectangular space where two further oval openings have broken through the roof, presumably from surface damage over the centuries. A large boulder sits on the chamber floor, and a possible second souterrain lies approximately 100 metres to the southwest, suggesting this part of Cork may once have had more going on below ground than above it.