Cruach Na Caoile lies to the north-west of Oileán Mhic Dara, a island better known for its own early Christian associations, and at its south-eastern end the second edition Ordnance Survey six-inch map of 1899 marks a ruined rectangular church alongside a holy well sitting immediately to its south-east. Holy wells in Ireland are typically ancient water sources attached to a local saint's cult, often used for pattern days and votive offerings, and this one is paired with a church whose life as a functioning building had already ended long before the cartographers arrived.

The dedication is to St Coelann, also rendered as Keelan, a figure obscure enough that the island itself seems to have quietly absorbed her memory. The historian Roderic O'Flaherty noted the existence of a chapel here in 1684, recording its dedication in work later published by Hardiman in 1846. But by the time the Ordnance Survey Letters were compiled, the picture had already changed; the surveyors reported flatly that there was no chapel there at that point. What happened in the interval is not recorded. The structure may have collapsed through neglect, or its stones may have found their way into other uses, as happened with countless small island churches along this coastline. What the 1899 map captured, then, was not a building in use but a footprint, a rectangular outline that the mapmakers dutifully recorded even as it disappeared back into the ground.