Beneath a field that once formed part of Mr Lyne's land, an underground stone chamber lay sealed and forgotten until someone, at some point before the 1930s, disturbed a large flagstone and found themselves looking into the past. Today no surface trace remains, but the dimensions recorded at the time of discovery are precise enough to conjure the space clearly: an oblong chamber running east to west, roughly 3.66 metres long, 1.5 metres wide, and somewhere between 1.22 and 1.52 metres high, just tall enough to crouch in, its floor and walls filled with debris accumulated over centuries.

A souterrain is an underground stone-built passage or chamber, typically associated with early medieval settlement in Ireland, and often interpreted as a place of refuge, storage, or both. This one sits within what may be a rath, the circular earthen enclosure that was the standard farmstead of early medieval Ireland, though the enclosure at Coolies is itself uncertain. What was recorded more confidently in the 1930s, by a Capt. D. B. O'Connell, was the layout of the structure itself: a low square doorway, roughly 76 centimetres on each side, cut into the east end of the chamber's north wall, opening onto a ramped passage running north to south for about 3 metres, which was thought to lead toward the original entrance to the souterrain. The ramp suggests the entrance was once approached from ground level, descending into the chamber rather than simply crawling horizontally into it.