The structure known locally as Dermot and Grania's Bed is a wedge tomb, a type of megalithic burial monument built during the late Neolithic and early Bronze Age, typically consisting of a roofed stone chamber set within a cairn of smaller stones. The name itself belongs to a folk tradition that attached itself to ancient megalithic sites across Ireland, linking them to the legend of Diarmuid and Gráinne, the eloping lovers who fled across the country pursued by the warrior Fionn Mac Cumhaill. So many prehistoric monuments acquired this name that the pairing became a kind of archaeological joke; Ireland, it turned out, had enough beds to accommodate years of running.

This particular structure sits on flat limestone cragland in County Galway, set within a cairn measuring 22 metres in length and approximately 9 metres in width, aligned on a NNE-SSW axis. Towards its northern end, the cairn incorporates a well-preserved chamber formed of two tall portal-stones, two narrow sidestones, and a large backstone, the whole assembly covered by a substantial roofstone. The monument was documented in detail by Ruaidhrí de Valera and Seán Ó Nualláin in their Survey of the Megalithic Tombs of Ireland, published in 1972, which remains one of the foundational references for this class of structure. A related wedge tomb lies roughly 700 metres to the south-west, suggesting that this stretch of Galway cragland was, at some point in prehistory, a landscape of some funerary or ceremonial significance.