At sixteen metres long, one metre wide, and one and a half metres high, it is a tight, deliberate space, its walls built from nine courses of dry-stone and sealed above by flat capstones. Openings at both ends mean it is not a dead-end chamber but a through-passage, which raises quiet questions about how it was used and by whom.

This is a souterrain, a type of underground stone-built structure associated with early medieval settlement in Ireland, typically dating from roughly the seventh to the twelfth centuries. Souterrains are found across the country, often tucked inside or beneath ringforts, the circular earthen or stone enclosures that were the standard farmstead of the period. Their exact purpose is still debated; food storage, refuge, and simple shelter have all been proposed, and the answer may vary from site to site. The Rathcarreen example sits within its ringfort in the manner that is most commonly recorded, serving whatever household once occupied that raised ground above it. The dry-stone construction, without mortar, relies entirely on the careful placement of each course to hold its shape, and the fact that this passage survives structurally intact across its full length is itself worth noting.