From archaeology.ie:
A circular hollow (diam. 0.4m; D 0.25m) in a natural rock outcrop is known locally as the ‘Wart Well’. Traditionally, the ‘well’ appeared after St Brigid’s horse left a hoof-mark in the stone. For a cure, it must be visited three times; a pin is left at the first visit and at the third visit the pins and warts will have disappeared.