The friendly landowner convinced me not to go out to this site and it is completely overgrown. It's marked as a holy well on the map but as a bullaun stone on the National Monuments Database.
The lady said that there are some Americans interested in the site and that they are going to do some work there in the near future, tidying up the site and freeing up access. I asked could I come back next summer and she said sure.
(I've posted this here as the folklore is worth relating)
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.