The knowledge of its existence comes not from excavation or any physical feature you could point to, but from local information passed on and eventually recorded. It is, in a sense, a monument that survives entirely in memory.

A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlements in Ireland, often constructed beneath or beside a rath, the circular earthwork enclosure that served as a farmstead from roughly the sixth to the twelfth centuries. They were used variously for storage, refuge, or ventilation of dairy goods, and hundreds are known across Munster. The rath at Greenane is itself a recorded site, and local tradition holds that its souterrain was closed up at some point, leaving the underground structure intact but inaccessible. What sealed it, and when, is not recorded.