Known as St Kevin's Bed, it sits in the rock of Lugduff on the southern shore, reachable only by boat or by a precarious traverse along the cliff, and it has resisted easy categorisation for well over a century.

The traditional account is hagiographic: the sixth-century monastic founder St Kevin is said to have used the cave as a sleeping place, retreating into the rock face above the water as part of a life of extreme ascetic withdrawal. That association is old and persistent. But scholars have also proposed it as a Bronze Age burial site, a rock-cut tomb of the kind occasionally found in upland areas where natural outcrops were worked into funerary chambers. A third possibility complicates both readings. The cave may be the opening of a mine, and a second partial opening at a lower level hints that whatever was begun here was not finished, or at least not completed in a form that survives. Leask, writing in 1950, and Barrow, in 1992, both addressed the site without resolving the question, which is itself a measure of how genuinely ambiguous the physical evidence remains.

The Discovery Programme, an Irish archaeological research body, has produced a detailed three-dimensional digital model of the cave, allowing the interior to be examined in a way that the awkward cliff-side position makes difficult in person. For those who do visit Glendalough, the cave is visible from the lake shore, though accessing it directly requires a boat and some care on the rock; its position above the water gives it an undeniable atmosphere, whatever its original purpose.