I am still not sure if I found this stone. It is definitely no longer standing anyway. I found two very impressive candidates lying prone at the base of oak trees in the rough location. I believe that one of these is definitely the stone marked on the map as the oaks were the only deciduous trees in this conifer plantation.
Follow the R115 south from Dublin and park in the car park at O121 237 (there is a big sign saying Hell Fire Club). Right by the entrance to the car park there is a path leading up the hill, follow this about 50 m past the track that crosses it to a large stone. The two candidates I found are in the trees to the left.
This is an explanation of (and a bit of a disclaimer for) the coordinates I provide.
Where a GPS figure is given this is the master for all other coordinates. According to my Garmin these are quite accurate.
Where there is no GPS figure the 6 figure grid reference is master for the others. This may not be very accurate as it could have come from the OS maps and could have been read by eye. Consequently, all other cordinates are going to have inaccuracies.
The calculation of Longitude and Latitude uses an algorithm that is not 100% accurate. The long/lat figures are used as a basis for calculating the UTM & ITM coordinates. Consequently, UTM & ITM coordinates are slightly out.
UTM is a global coordinate system - Universal Transverse Mercator - that is at the core of the GPS system.
ITM is the new coordinate system - Irish Transverse Mercator - that is more accurate and more GPS friendly than the Irish Grid Reference system. This will be used on the next generation of Irish OS maps.