Tobar Cholmcille, as it is known locally, is not a spring or a pool in any conventional sense but a smooth, cylindrical depression worn into a slanting rock face, sitting below the high-water mark and exposed only at certain states of the tide. Its modest dimensions make the devotion it has attracted all the more striking.

The well takes its name from Saint Colmcille, the sixth-century monk and missionary also known as Columba, and its significance was already well established by the late seventeenth century. The Connacht historian Roderic O'Flaherty mentioned it in 1684, a reference later preserved in James Hardiman's 1846 edition of O'Flaherty's work on the region. The date that draws people here is the 9th of June, the feast day of Colmcille, and accounts suggest the well was widely known and much frequented on that day. The pattern of visiting a holy well on a saint's feast day, often involving prayers, ritual circuits, and sometimes offerings left at the site, was once common across Ireland and remains alive in a number of places. That this particular site sits in the intertidal zone adds an unusual layer of precariousness to the tradition; reaching it at all depends on the sea.