What they found was a souterrain, an early medieval underground stone structure, typically used for storage or refuge, consisting of two narrow chambers arranged at right angles and connected by a low creepway through which a person would have to crawl. The discovery was subsequently investigated by J.P. McCarthy, whose findings were published in 1977.

The two chambers are stone-built and roofed with horizontal lintels, large flat stones laid across the top. The first chamber runs roughly east to west, measuring 4.4 metres long and just 0.8 metres wide, with a maximum height of 1.2 metres. Its floor slopes downward towards the west, and its end walls are rounded rather than squared off. The second chamber, oriented north to south, is slightly larger at 4.9 metres long and 1.5 metres high, and has an air vent set into its rounded northern end. McCarthy suggested that the original entrance to the whole structure was at the eastern end of the first chamber, where the ground rises towards the surface. Access today is through a gap in the roof of that first chamber, where only two of the original lintels remain in place. What makes the Ballyanly souterrain particularly arresting, though, is what was found inside. The first chamber had been deliberately backfilled, and within that fill were human skeletal remains, along with slag, furnace bottoms, an iron ring, and an animal tooth. The slag and furnace bottoms point to ironworking somewhere nearby, and the careful backfilling over the skeletal remains suggests the deposit was not accidental. Whether the chamber was repurposed, ritually closed, or simply used for burial after its original function ended is a question the finds alone cannot fully answer.