I found reference to this headstone while parked in the tiny village of Glenroe (which actually does exist). Another time while passing through this area I spotted the sign for the graveyard itself and took a spin to see it. Its not far from the crossroads that is marked with a sign Darragh.
The art itself consists of a cross, a backwards P and a long snaky symbol.
The info board has this to say about it "A large stone, resembling a headstone, which may also be seen in the graveyard, appears to have 'Rock Art' (abstract images carved on stone), which may also date from the Megalithic or Neolithic periods"
I think this is unlikely however and it may more likely be Christian art.