Refereeing matches when any of the ol the old firm are involved has always been a problem for refs, especially at Celtic park and Ibrox as they are intimidated by the home support.
I remember a few years ago, a friend of mine, playing a premier game at Ibrox, against rangers. His side were beaten 2-0 and both goals were offside. This was proved by the cameras after the game.
After the second goal, my friend ran to the offending stand - side linesman to protest, the linesman said two words to him,
Phuck off !! , so much for the officials !
I believe Bob Valentine used to book rangers players with a green pencil and Celtic players with a blue pencil !