It sits on rocks exposed at low water, hidden beneath masses of seaweed, and takes the form of a natural pothole, a smooth hollow worn into the rock over time by water and wave action rather than carved by any human hand. It has never, as far as the record goes, been formally visited by a researcher.

Locally it is known as Tobar Cholmcille, the well of Colmcille, the sixth-century saint who founded the monastery on Iona and whose name is attached to sacred water sources across the west of Ireland and Scotland. Holy wells dedicated to him tend to carry associations with healing and with the older, pre-Christian habit of venerating water that was simply absorbed into Christian practice rather than replaced by it. The identification of this particular pothole as his well was recorded by Tim Robinson in 1985, that meticulous mapper of Connemara and the Aran Islands whose fieldwork captured the local names and traditions that might otherwise go unrecorded. The hollow itself is natural, which is not unusual; many holy wells across Ireland are unremarkable in physical terms, their significance residing entirely in what generations of people believed about them.

Reaching it would require timing a visit carefully around the tides and navigating the exposed eastern rocks of the island, where the seaweed covering the pothole would make it easy to walk straight past without knowing what you were looking for.