The one at Bawnlahan townland in County Cork came to light only by accident, in 2001, when topsoil removal displaced several of its roof lintels. What emerged from the disturbance was something considerably more complex than a simple tunnel: a branching underground system of chambers and narrow connecting passages, cut partly into earth and partly into rock, sitting on a west-facing slope above Tragumna Bay.

The layout, as far as it could be traced, opens with an irregular main chamber roughly 2.6 metres by 2.2 metres, with a surviving height of just 1.25 metres. A single cracked lintel remains in place; others have fallen. From this chamber, several narrow creepways, passages low enough to require crawling, lead off in different directions. One to the south-east is blocked by collapse, another to the south similarly so. A third, at the west side, connects to a longer earth-cut passage, nearly five metres in total, roofed towards its far end by four large flat lintels supported partly by stone walling and partly by a single upright stone set on a ledge of uncut earth, separated from the lintel above by two small pad-stones. Beyond this point the floor rises sharply, then more gradually upward to meet the roof, which may represent the original entrance to the whole structure. A further creepway off this passage leads down into a circular or oval rock-cut chamber, which itself connects to at least one more chamber beyond. Those deeper spaces could not be safely entered or examined at the time of investigation, so their extent remains unknown.

The structural detail here rewards close attention. The combination of earth-cutting and stone construction, the staggered levels between the main chamber and some of its connecting openings, and the presence of multiple creepways, all suggest a deliberately designed complexity rather than simple excavation. Souterrains of this type are found across Ireland and are generally dated to the early medieval period, though without excavation it is difficult to assign a more precise date to the Bawnlahan example. The partial collapses throughout suggest the structure has a long history of gradual deterioration, and the 2001 discovery likely accelerated exposure to the elements.