Roughly thirty metres long, over two metres high, and built entirely by hand from dry-stacked stone in the corbelled or beehive style, it opens through the southern face of the cliff and runs inland in the direction of the castle before a cave-in blocks further progress. A second branch simply ends, sealed off with no apparent exit. It is, in other words, a souterrain, an Irish term for these deliberately constructed underground passages and chambers, typically associated with early medieval settlements, where they served variously as storage spaces, refuges, or escape routes. What makes this one unusual is its setting: it sits beneath a promontory fort, that is, a defensive enclosure using a coastal headland as natural protection on multiple sides, with the cliff edge doing work that a rampart elsewhere would have to do.

The castle above dates to the later medieval period, occupying the highest point of the promontory. Whether the souterrain and the fort predate it by centuries, as is common in sites of this type, or whether the tunnel served some function connected to the castle's occupation, the notes do not say. What is clear is that the passage was unearthed in 1987, the year being significant in that it suggests discovery during some form of work rather than a planned excavation. The construction method, corbelled stone forming a chamber of more than two metres in height, implies considerable effort and skill, and the scale of it is larger than many souterrains found elsewhere in Ireland. The cul-de-sac branch in particular raises questions that remain, as far as the available record goes, unanswered.