The whole thing was so narrow that for most of its length a person could neither stand upright nor turn around. It was not the kind of thing that turns up when gardening.

The structure was examined in 1962 by the National Museum of Ireland, whose investigator recorded it in considerable detail. The tunnel was built of local limestone, with vertical dry stone walls carefully dressed to an even surface and, for much of its length, plastered with mortar. Flat lintels, averaging 60 centimetres measured along the passage, formed the roof throughout. The dimensions were tight: at the narrowest surviving section, roughly five metres from the house end, the passage was only 45 centimetres wide at the floor and 48 centimetres high, widening gradually as it descended eastward beneath the slope of the ground. Three branches led off the main axis, but all had been deliberately blocked with large loose stones at some point, and it was impossible to determine whether they continued beyond their filled sections. The terminal chamber was a different proposition entirely: a rectangular room three metres by 1.7 metres, with two walls of natural rock face and a vaulted roof of mortared stone carried on wooden beams, whose shapes survived in the mortar long after the timber itself had rotted away. Animal bones, fragments of 18th-century wine bottles, and glazed ware were found on the chamber floor. The occasional brick incorporated into the tunnel walls, the liberal use of mortar plaster, and the 18th-century pottery all pointed the investigator toward a relatively late date for the construction, though no firmer conclusion could be drawn. What the structure was actually built for, whether for storage, concealment, drainage, or something else entirely, was left open.