It was not built recently. The passage is a souterrain, an underground tunnel constructed from large flat stones forming a roofed corridor roughly a metre wide, of the kind typically associated with early medieval settlement in Ireland, likely used for storage or refuge. What makes its discovery particularly striking is that it runs directly under the Multyfarnham road, meaning that traffic has been passing over an ancient underground structure without anyone necessarily knowing it was there.

The souterrain came to light in January 1999, when archaeologist Sylvia Desmond, working for Judith Carroll and Co. Ltd under licence 99E0036, excavated eleven machine-cut test trenches across a site where two houses were planned. The exact location of the known souterrain monument was uncertain before the work began, and the trenches were intended to clarify matters before construction proceeded. The six trenches opened for the northern house revealed not only the souterrain passage but also paving, deposits of charcoal, and linear features cut into the natural boulder clay, some filled with bone and occupation debris, suggesting prolonged habitation. More significantly, the overall pattern of features indicated that the souterrain sat within a ringfort, a type of circular enclosed settlement common across early medieval Ireland, typically defined by an earthen bank and external ditch. The southern bank and ditch of this example survived reasonably well, the bank standing to 0.6 metres in height and 2 metres wide, with a ditch just under a metre wide beside it. The northern and western portions were harder to read; the ringfort appears to extend beneath a standing cottage and possibly across the road itself. Its estimated dimensions within the excavated area ran to approximately 38 metres north to south and 30 metres east to west. The ringfort had not previously been recorded at all.