What is your theory Cian?
That the passage-tomb builders reused rock art that was lying about the countryside as part of a new culture? Would that mean that the passage tomb builders didnt create the art that adorns them?
(Not being smart actually quite the opposite.)

Ken , I agreee that ,the stone in the pic is atypical compared with the rock art found in other passage graves and elsewhere in the Loughcrew cairns , but the others are unlike the usual found in the open air even having a quite distinct style .It is maybe no coincidence that the stone is not in a passage and compares with Newgrange where the open air motifs , cups and circles are found on the exterior kerb and and rarely in the passage or chambers .
Deciding whether the carvings were done "for the job " is difficult to prove either way , fwiw , I think the majority were on order but there are later cases like Clava where the marked rocks were probably from a much earlier marked outcrop and saved doing a new one , like Newgrange they certainly wern't carved in situ . I would extend the use of carved rocks from outcrops to use as cist covers , Craig Hill has obvious signs of quarrying as do other sites possibly providing an al frsco cist cover factory .
The dating of RA world wide is showing that is much older than previously believed e.g. Lower Paleolithic so Early Neolithic is almost modern .Clive Waddington suggets Mesolithic but with little evidence .