Its upper surface, level with the surrounding earth, holds two roughly circular hollows, each about twenty centimetres across and twelve centimetres deep, along with a smaller pointed depression. The hollows are said never to dry out, even in the height of summer, and it is this quality, as much as anything, that has kept people coming back to them for generations. The stone is not a well in any conventional sense; it is a bullaun stone, a type of ancient carved or naturally worn rock found across Ireland that was frequently absorbed into Christian devotional practice, its depressions filling with rainwater that came to be regarded as holy.

The saint whose name the site carries is Mochuda, also known as Carthage, associated with the early medieval monastery on Innisfallen island in Lough Leane. According to legend, the saint was drawn away from Innisfallen abbey by the singing of a bird, followed it into the woodland, and fell asleep beneath a tree, where his knees and chin left their marks in the stone. In the schoolchildren's version of the story, collected in Killarney in the mid-twentieth century, the sleeper is an unnamed monk who slumbered for a hundred years, woke to find everything changed, was recognised only by the memory of an old tale, and died quietly among the younger monks. By the 1840s the knee hollows were already being explained as the saint's impressions, and a chin mark noted by a researcher in 1960 extended the story further. The site was at some point enclosed within Kenmare Demesne, the old public path closed by grand jury order, with a caretaker at Deenagh Lodge holding the key.

The ritual once practised here was unusually precise. Pilgrims arrived before sunrise, bringing water in a vessel to replenish what they used from the stone, three rags to hang on the overhanging tree, and fragments of bread. The sign that a request had been granted, or a cure effected, was the appearance of a robin on the tree. One account, told from personal experience, describes how three women in a group of four saw the bird on the third morning; the fourth, who had come hoping for a cure, died not long after. The stone is still visited each May.