Just for the record ... The stones in Carlow are all around 52' 50'', so any relationship there can probably be ruled out.

On reflection, only one of the four visited today actually looked as though it had been 'worked' and in particular pair of stones only one had the grooves in ... and yes they were of different stone.

What struck me as odd though was that they are all tall thin stones like the devils arrows. None of the 'squat' stones in the area show these markings. This would lead me to think that they chose a different sort of stone for different styles of monument.

(Oh ... and in fearof rambling too much)
I also came across a man made mound, about 15m high, with a lovely stone stuck right on the top.

Thanks once again for the hints and info.