At Cahernagollum in County Mayo, a megalithic cist, a box-like burial chamber formed from upright stone slabs, emerged from within a cairn when the surrounding stone was being removed for other purposes. What the quarrying exposed has remained: two large side stones, one leaning towards the other at an angle, and a capstone roughly 1.2 metres long, 1.5 metres wide, and 0.3 metres thick resting across them. The chamber stands about 1.3 metres high and is oriented east to west, an alignment common in prehistoric funerary monuments across Ireland.

Cists of this kind typically date to the Neolithic or Early Bronze Age, built to contain the remains of the dead along with personal objects, pottery, or tools placed there for whatever passage was imagined to follow. What makes this example quietly notable is precisely what was absent: no burials and no grave goods were recorded when it came to light. Whether the chamber was robbed out, emptied long before quarrying began, or perhaps never used in the conventional sense, is not known. The cairn that once enclosed it, a mound of loose stone piled deliberately over the tomb, would have protected the chamber for thousands of years before the quarry work brought it into the open.