Known locally as St Kevin's Cup, it is a bullaun stone, a type of prehistoric or early medieval rock feature in which a deliberate circular hollow has been ground or carved into the surface. Such hollows are found widely across Ireland, often near ecclesiastical sites, and are associated in folk tradition with saints and healing waters. This one, roughly squared and measuring less than a metre in length, has a single basin set slightly off-centre towards its northern side, around 36 centimetres across and 14 centimetres deep.

The stone sits about 30 metres east of the statue of the poet Sir Thomas Moore, who gave the Meeting of the Waters its literary fame through his early nineteenth-century song of the same name. The boulder itself shows a fracture along its eastern side that appears relatively recent, suggesting the stone has had a complicated physical history even if its ritual associations reach back much further. The connection to St Kevin, the sixth-century monastic founder of Glendalough, is typical of the way early Christian saints came to absorb older sacred sites and objects into their own stories. Whether the cup-shaped hollow ever held water used in devotional practice, as many bullaun basins are said to have done, the notes do not say, but the pairing of a saint's name with a river confluence is seldom accidental in the Irish landscape.