A souterrain is a dry-stone underground passage or chamber, typically associated with early medieval settlement in Ireland, often used for storage or as a place of refuge. What makes this one particularly striking is not only how close it lies to the surface, but that it was effectively unknown until a planning application for two houses brought it to light.

In January 1999, archaeologist Sylvia Desmond, working for Judith Carroll and Co. Ltd under licence No. 99E0036, excavated eleven machine-cut test trenches across the site. Five trenches in the southern portion of the site turned up nothing at all, reaching natural boulder clay just 30 centimetres down. The six trenches to the north told a different story entirely. The souterrain passage, 1 metre wide and roofed with large flat stones measuring roughly 30 by 20 centimetres, was traced running the full width of the proposed building plot, and was clearly continuing beyond the site boundary under the Multyfarnham road itself. Associated features within the trenches included linear cuts filled with charcoal and bone, areas of paving, and what may have been occupation pits. The excavation also revealed that the souterrain sat within a previously unrecorded ringfort, a type of enclosed early medieval farmstead typically defined by earthen banks and ditches. The southern bank and ditch of this enclosure were well-preserved, the ditch running just under a metre wide and 60 centimetres deep, while the bank survived to a height of 60 centimetres. The full extent of the ringfort was harder to pin down; the northern portion appeared to extend into the garden of a neighbouring cottage and possibly across the road as well. Within the area assessed, the ringfort was estimated at approximately 38 metres north to south by 30 metres. A small quantity of animal bone and some sherds of post-medieval pottery were among the finds recovered.