I remember, when I was young, visiting Gougane Barra and being amazed at the coins hammered into the tree and then last year seeing a sign requesting that coins not be placed in the tree.
Until recently, I had never seen coins hammered into a rock.
These were taken on a walk into Derrymore Glen, near Camp on the Dingle Peninsula when the rock was used as a table for a food and drink break.
The web does indicate the practice of hammering stones into a mass rock in Armagh as a type of offering.
Maybe that is what I saw in Derrymore. Or maybe it is something completely different.