This is a boulder-burial, a form of prehistoric monument found mainly in south-west Ireland, in which a single substantial capstone is placed over a small number of upright support stones to create a low, table-like structure. The cover-stone here is subrectangular, measuring 2.7 metres by 1.7 metres and roughly 0.9 metres thick, which makes it a considerable mass of rock by any reckoning. The problem is that it has shifted from its original position, and now rests across three stones beneath it, only two of which are thought to be the original supports. Something, at some point, disturbed the arrangement.

Boulder-burials are generally understood to date from the Bronze Age, and their purpose, as the name suggests, is funerary, though the precise rituals associated with them remain a matter of archaeological debate. They tend to be low-profile monuments, easy to miss in a landscape, which may partly explain why so many have been disturbed over the millennia. The Inishbeg example occupies an quietly unusual position, situated on an island at a river mouth, a location that would have carried its own significance in prehistoric thinking about boundaries, transitions, and the movement between land and water. Whether the displacement of the capstone was the result of deliberate human interference, gradual subsidence, or some other cause is not recorded.