Situated near the end of the southern rath is a large rock that has been partly destroyed. The top displays 18 cup marks. I imagine that there must have been a lot more on the missing part as some of them have been cut through.
Without the rest of the stone it is hard to be certain, but I think the marking might display a pattern. I ripped my trousers quite impressively while going back to the car to get some water to help make the cups show up better. Typical!
From Donard head north towards the Hollywood Glen.700m out of Donard you will cross a bridge and passa right turn. There is a modern stone gateway on the left. Shortly after this you will cross a stream (but via no obvious bridge). Stand at this crossing point and look into the field on the right hand side of the road. If you look left along the hedge inside the field you will see the raths. Just before them is a large stone. This is what you are after.
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.