What makes it unusual is not its size, though eighteen metres across is considerable for a natural pool, but its classification: this is a holy well, one of thousands scattered across Ireland, where natural water sources were venerated long before Christianity arrived and were later absorbed into folk-religious practice. Most people associate holy wells with carved stone basins, votive rags tied to nearby branches, or the name of a patron saint carved into a kerb. This one presents itself differently, as an unadorned natural feature, level with the marshy ground around it, with little to mark it out to a passing eye.

Holy wells in Ireland occupy a curious overlap between pre-Christian ritual and Catholic devotion. Water emerging from the ground was understood for centuries as having curative or protective properties, and many wells became sites of pattern days, local festivals held on a saint's feast day that combined prayer with communal gathering. The Tonlegee well leaves no documentary trace in the available record beyond its physical description, which is itself spare: a natural pond on low, wet ground, feeding a moving stream. That simplicity is not unusual for wells in the west of Galway, where the landscape itself can be the monument.