The structure is a souterrain, an underground passage or chamber built during the early medieval period, typically beneath a rath, which was a circular earthwork enclosure used as a farmstead. What makes this one quietly puzzling is that by the time cartographers were recording the area in detail, the souterrain had apparently been forgotten as such. The nineteenth-century Ordnance Survey six-inch map does not mark it at all, and later editions of the twenty-five-inch plan label it simply as a well.

The souterrain sits within a rath and opens through a lintelled gap on its eastern side, the opening just over a metre high and nearly one and a half metres wide. Inside, the passage is short, roughly one and a half metres long, with corbelled upper courses, where the top stones project slightly inward to carry the weight of the roof slabs above. Soil has slumped in from the east end, and a large loose slab, probably a displaced lintel, appears to have been placed relatively recently across a low gap at that end, effectively blocking it. The walls continue westward beyond the roofed section for about two metres before disappearing into vegetation and loose soil. The landowner notes that water collects naturally at the base during wet weather and winter months, which may explain why later generations recorded it as a well and why a low encircling wall of rough stones, now little more than a pile forty centimetres high, was built around it at some point to mark the spot and keep livestock away. Whether the structure was deliberately converted to a water source or simply became one as it collapsed and silted up, the two-stage life implied by the cartographic record gives it an understated oddness.