It was an accidental rediscovery of the best kind: the machine punched through one of the stone lintels, opening a small gap into a space that had not been marked as anything more than a probable site since at least 1948, and had shown no surface trace whatsoever when someone came to look for it in February 1984.

A souterrain is a man-made underground chamber or passage, typically built during the early medieval period in Ireland, constructed from drystone walling and roofed with large flat stones. They are found across the country in association with settlements and are thought to have served as cold storage, places of refuge, or both. The Stonyisland example is a modest one: 4.5 metres long and between 1.2 and 1.55 metres wide, aligned north to south, and sitting nearly two metres below the surface of what is now undulating pastureland. Its walls were built of random coursed drystone limestone, battered slightly inwards as they rose, with a single course of corbelling at the top to carry the weight of five flat lintels. When inspectors were able to peer inside through the aperture the digger had created, they found the passage filled with loose clay and compressed to a height of just 25 centimetres at its northern end, opening to around half a metre at the south. There was no sign of a creep, the low connecting passage that sometimes links one souterrain chamber to another, and no second chamber at all. The trench was to be backfilled after inspection, with the souterrain itself left undisturbed by the development works.