Beneath the ground lay a souterrain, an underground stone-built structure of the kind typically associated with early medieval Ireland, used variously for storage, refuge, or both. What made this particular discovery unusual was not just its survival intact beneath a modern development, but what it contained and how meticulously it had been put together.

A souterrain, for those unfamiliar with the term, is a deliberately constructed underground passage or chamber, built from dry stone and often associated with early Christian period settlements. The Beaufort example consists of two stone-built chambers connected by a low creepway, a narrow linking passage through which a person would have had to crawl. The chambers themselves were built with walls that angle slightly inward as they rise, a technique known as batter, which adds structural stability. The south-east end of the first chamber is corbelled, meaning the roof is formed by progressively overlapping stones rather than flat slabs, a precise and laborious method of construction. Both chambers were roofed with stone lintels, four of which remained in position when excavated in the early 1990s; two others lay on the ground nearby. Follow-up excavation in 1995, before the souterrain was resealed, uncovered a steeply rising unroofed entrance passage at the north end of the second chamber. Tucked into the wall of this entrance passage was a tanged iron sickle, a single-bladed agricultural tool dating to the early medieval period, placed deliberately between two stones rather than dropped or lost. Whether it was concealed for safekeeping or left there with some other intention is impossible to say now, but the gesture has a quiet deliberateness to it that tends to stay with you.