A shallow depression, roughly two metres east to west and just over a metre north to south, sits slightly north-west of centre within the enclosure. At its western end, a lintelled opening, less than a metre wide and only sixty centimetres high, leads downward into darkness. What lies beneath is a souterrain, an underground passage of dry-stone construction, the stones laid without mortar, relying on careful placement alone to hold their shape across the centuries.

Souterrains are a recurring feature of early medieval Irish settlements, typically associated with raths, the circular earthen enclosures that served as farmsteads for Irish families from roughly the sixth to the twelfth centuries. They were used for cold storage, for refuge, or sometimes both, and their narrow entrances were a deliberate feature, making pursuit difficult. The one at Shanaghy appears to run westward from that interior opening and most likely connects to a second lintelled gap visible in the outer face of the rath bank, some ten metres to the south-west. That second opening is smaller still, only fifty centimetres wide and thirty centimetres high, an exit that would require a person to crawl through almost flat. The passage between the two has not been fully excavated or recorded in detail, but the alignment and the spacing are consistent with a single connected underground route running beneath the bank itself.

What makes this particular example quietly interesting is the way the two openings, one inside the enclosure and one outside it, suggest the souterrain was designed as much for escape as for storage. The outer exit, hidden in the bank face and barely large enough to squeeze through, would have been very difficult to locate quickly from outside. Whether the rath at Shanaghy was ever called upon to test that design is unknown.