The well on Oileán Ros na nEas is not a constructed shrine or a tended grotto but something far more modest: a natural pothole, roughly thirty centimetres across, set into a west-facing slope of bare rock. That the site was ever noticed, named, and incorporated into devotional practice at all says something about the particular intensity with which the Irish landscape was once read for spiritual significance.

The well carries at least three names, each telling a slightly different story. Tobar Cholmcille links it to Saint Colmcille, the sixth-century monk and missionary whose feast day falls on 9 June, precisely the date on which the well was traditionally visited. The more unusual name, Tobairín Rí an Domhnaigh, translates roughly as the little well of the King of Sunday, a phrase with a distinctly archaic and liturgical feel. It is also known simply as Tobar an Oileáin, the well of the island. Patterns at holy wells, the traditional rounds of prayer and circumambulation performed on a saint's feast day, were once a central feature of local religious life across Ireland, and this one, reached only by crossing to a tidal island, would have demanded a degree of effort and timing from those who observed it.

The island lies in the An Máimín area of west Galway, and because the well sits at the high-water mark on the south-western shore, access would depend entirely on the tides. The pothole itself is small enough to be easily overlooked if you did not know to look for it in the rock of the ridge.