It looks unremarkable now, repositioned during field clearance in the 1970s, but its original purpose was anything but ordinary. The stone was the capstone of a souterrain, a type of man-made underground chamber or tunnel built in early medieval Ireland, typically used for storage, refuge, or both. The passage it once sealed still lies beneath the ground nearby, unseen and largely forgotten.

The souterrain sits at the centre of a rath, the circular earthwork enclosure that was the standard form of a farmstead in early medieval Ireland, roughly between the sixth and twelfth centuries. Raths, sometimes called ring forts, were defined by one or more banks of earth and ditch, and the family or household living within them would often have had a souterrain beneath their dwelling or yard. The passage at Farranaspig followed this pattern, positioned centrally within the enclosure. When the covering stone was removed during farm clearance work in the 1970s, it was not discarded but shifted to the roadside, which at least preserved it in the landscape, if not in its original context. The capstone's dimensions, three metres in length, 1.7 metres wide, and 1.2 metres high, suggest this was a substantial slab, likely requiring considerable effort to move even with modern equipment.