Benchmarker refers to this type of 'mark - one carved into an undressed stone - as a 'stoney'. This one is near the top of Infirmary Road in Dublin.
Below: The stoney on Infirmary Road.
Above: Infirmary Road looking north towards the Phoenix Park entrance gate on the N.C.R. with the stoney at bottom left. Standing by and mulling whether to jump on a bus to Dun Laoghaire, is M.C. who on his many childhood jaunts along that road from his nearby home had never previously set foot on that side of it. Strange but true. Apparently.