Somewhere beneath the pasture at Ballynakilla, a network of underground stone passages was discovered during land reclamation works, then filled in again, and the ground closed over it as if nothing had ever been there.

Souterrains are underground stone-lined passages and chambers built, usually during the early medieval period, in association with settlements. Their precise function is still debated, but they are generally understood to have served as places of refuge, cool storage, or concealment. What was reportedly found at Ballynakilla was unusual even by the standards of these structures: local accounts described not a single passage but "many underground passages," suggesting something more complex than the typical single-chamber arrangement. The discovery came not through archaeological investigation but as an incidental result of land reclamation, which means no systematic record of the layout or construction was made before the decision to fill it in for safety reasons. That decision, practical as it was, closed off whatever the passages might have revealed about the people who built them and used this slope overlooking the harbour.