A souterrain, the term for an underground stone-lined passage or series of rooms typically associated with early medieval settlement, once opened onto the southern side of a rectangular enclosure here. The enclosure itself has since been levelled. The entrance to the underground system was discovered outside the old ramparts, and was subsequently infilled by the landowner. There is no visible surface trace of any of it.

What makes the place linger in the mind is an account left by the antiquarian Grove White, writing between 1905 and 1925. He described what he found as an oblong fort with chambers underneath, reached by steps leading down into them. Inside, there was an arched passage choked with a dark material that looked like soot, which he attributed to cremation. It is a small, specific detail, the kind that resists easy interpretation. Souterrains are generally understood as places of storage or refuge, sometimes connected to ringforts and other enclosures across early medieval Ireland, but the presence of material suggesting burning, possibly the residue of funerary practice, complicates any tidy reading of this one. Whether the chambers served multiple purposes over time, or whether Grove White was encountering something deposited long before the souterrain's primary use, is not something the surviving record can answer.

The enclosure to which the souterrain belonged has been assigned a separate record and was itself already levelled by the time the site was formally documented. The underground chambers, once accessible, are now sealed. The field at Lisnagoorneen holds no markers, no trace above ground of the steps, the arched passage, or whatever left that dark residue behind.