This is a souterrain, an underground chamber or gallery, typically of early medieval date, built from dry stone and used for storage or as a place of refuge. What makes this one quietly puzzling is the question of whether it is one feature or two. The antiquary T. J. Westropp, writing at the turn of the twentieth century, recorded two souterrains at the site, and his plan shows two separate cave-like features. Yet his written description of the cashel tells a different story, mentioning only one. The two features on his plan sit close enough together that the whole arrangement may simply be two sections of a single structure.

Westropp documented the cashel across several publications between 1896 and 1913, and his notes remain the earliest detailed record of what he found there. When the site was inspected again in 1997, surveyors recorded one souterrain in the south-western quadrant of the cashel. The passage runs on a roughly south-south-west to north-north-east alignment. At its widest it reaches just under one and a half metres, and at its highest point around one and two-thirds metres, so a person of average height would need to stoop to move through it. The walls are corbelled, meaning the stones are laid so that each course projects slightly inward over the one below, gradually narrowing the space until the roof is reached. Four stone lintels close the passage overhead. At its far end, the passage terminates at a small stone bench, just thirty centimetres deep and fifty centimetres high, whose precise function remains unclear. The cashel also contains the remains of a house site within its enclosure, suggesting this was once a place of some domestic complexity.