Discovered in 1985, this souterrain, an artificial underground passage or chamber system typically built during the early medieval period, sits in the south-western quadrant of an early ecclesiastical enclosure. That combination, a hidden subterranean structure tucked within the grounds of an early church site, places it among a particular class of monument that archaeologists and historians still argue over. Were souterrains used for refuge, for storage of perishables, for ritual purposes, or for some combination of all three? No single answer has settled the debate.

The structure at Carrigroe consists of three earth-cut chambers arranged in sequence. The original entrance appears to have been a lintelled creepway at the south-western end of the first chamber, a low, stone-roofed passage just wide enough to pass through on hands and knees, a design feature common to souterrains across Ireland and one that would have made uninvited entry considerably more difficult. Chamber 1 measures roughly 4.85 metres in length, 1.6 metres wide, and 1.25 metres high, a space tight enough to be claustrophobic but large enough to serve a practical function. Chamber 2, at 4.2 metres long and 1.5 metres wide, retains evidence of construction shafts in its north wall, vertical cuts made from above during the original building process and then sealed. Chamber 3 is now ruinous, and the current point of access into the system is through the collapsed roof of Chamber 2, the original entrance having long since become impassable. The details were communicated by R. M. Cleary and recorded in the Archaeological Inventory of County Cork.

The collapsed state of Chamber 2's roof, now the de facto way in, gives a reasonable sense of how vulnerable these structures are once the earth above them shifts or settles. The ecclesiastical enclosure in which the souterrain sits would originally have defined a sacred or monastic precinct, and finding an underground complex within such a boundary raises questions about who built it, who used it, and what relationship it had to the religious community nearby. Those questions, for now, remain open.