The entrance to a souterrain, a man-made underground passage typically built during the early Christian period as a place of refuge, food storage, or concealment, has been blocked up, and the earthwork that originally enclosed it has been levelled almost entirely out of existence. What remains is, essentially, a sealed door to a buried room that the landscape has done its best to forget.

The souterrain sits within the northern sector of what was once a rath, a circular enclosure defined by earthen banks and ditches that served as a farmstead in early medieval Ireland, roughly between the fifth and twelfth centuries. These enclosures were common across the Irish countryside, and it was not unusual for their occupants to construct a souterrain beneath or beside the living area, accessible from within the protected space. At Ballylin, the rath itself has been levelled, most likely through centuries of agricultural activity, leaving only the faintest trace of its form. The souterrain, recorded by McCaffrey in 1952, appears to run east to west, though with the entrance blocked and no excavation on record, the full extent of the passage remains unknown.