This is a souterrain, a type of roofed underground structure built during the early medieval period in Ireland, typically for storage, refuge, or both. What makes this one quietly unsettling is its orientation: rather than leading inward to safety, it leads outward toward a steep scarped bank, as though the builders were as interested in what lay beyond the edge as in what lay within the enclosure.

The souterrain sits inside a cliff-edge fort, a promontory-type enclosure that would have used the natural drop of the land as part of its defensive perimeter. The underground structure itself is modest in scale but carefully made. An open trench, roughly 4.5 metres long, 1.4 metres wide, and 0.8 metres deep, slopes downward from north to south into a roofed passage measuring around 3.1 metres in length, 1.65 metres wide, and 0.9 metres high. Large stone lintels span the passage, resting directly on the earth-cut sides rather than on built walls, and some of these lintels are described as being in a precarious state. The immediate surroundings add further texture to the site. About 30 metres to the north-north-east lies a bullaun stone, a boulder with one or more cup-shaped depressions ground into its surface, objects whose precise purpose remains debated but which are associated with early Christian and pre-Christian activity across Ireland. Some 25 metres to the north-west is a corn-drying kiln, a feature common to early medieval farming settlements, used to dry grain before milling or storage. Together, these three features suggest that the fort at Milleens was not simply a defensive structure but a working settlement with its own small agricultural and ritual landscape.