A shallow depression in the ground is the only surface hint of what might be below, and even that is uncertain. What was once described as several apartments communicating by narrow passages has, over the centuries, collapsed or silted over to the point where only a slight dip in the earth suggests anything is there at all.

The account comes from Samuel Lewis, who in 1837 recorded the discovery of these underground rooms within an ancient rath on the Greenville estate. A rath, in Irish archaeological terms, is a circular earthen enclosure, typically dating from the early medieval period, that served as a farmstead or homestead. The souterrain, an underground stone-lined passage or chamber system built within or near such enclosures, was a common feature of early medieval Irish settlement. They were used variously for storage, refuge, or both. The Greenville example sits within just such an enclosure, close to its southern bank, and Lewis's description of communicating apartments suggests a fairly elaborate construction, though no detailed investigation of it appears to have been carried out in any documented way. Whether the chambers remain structurally intact underground is unknown.