That depression, roughly 37 centimetres across and less than 10 centimetres deep, was not made for grinding grain or collecting rainwater. According to local tradition, it was filled with milk and food by the women of the valley, left there for the lunatics who had travelled, sometimes from considerable distances, to seek a cure in Gleann na nGealt, the Valley of the Mad. Two further hollows in the stone, apparently natural rather than carved, are said to be the handprints of those who drank from it.

A bullaun stone is a boulder or outcrop bearing one or more deliberately cut cup-shaped depressions, found at many early Christian and prehistoric sites across Ireland, though their precise function is often debated. This particular example, known in Irish as Cloch na nGealt, sits about 30 metres west of the Glannagalt river, close to an ancient ringfort. Its role was bound up with a wider landscape of healing. The valley contained two holy wells called Tobernagalt, the lunatics' well, and the route to them was marked by a ford across the stream known as Ahagaltaun, the madman's ford. The tradition of those suffering mental illness making their way here to drink the water and eat the watercress growing along the margins can be traced back in written record to at least the seventeenth century, as noted by the scholar De Brún. In 1869, P.W. Joyce set down a vivid account of the practice, writing that the water and the cress, together with "the secret virtue of the valley, will restore the poor wanderers to sanity."

The stone lies in quiet company with the ringfort to its west and the river to its east, in a valley whose Irish name has preserved the memory of its unusual history even as the practice itself faded. The two natural hollows described as handprints are worth looking for once you find the stone, small details that suggest how closely the physical object was once woven into the ritual life of the place.