Protestants hold in high regard King William of Orange. Therefore to a Celtic fan, they may be "Orange Bastards."(or basturts to use the local dialect).
Add the word "Dirty" to the beginning of the phrase and you have, "Dirty Orange Bastard" the initials of which form "DOB", which is where the widely-used Glasgow phrase "Dobber" originates from. Which, yes, means that if you're a dirty Rangers fan, and you call someone a dobber, you're slagging yourself. Up ye, and mon the hoops!
1a. England:- A 'dobber' is a large marble (small glass coloured balls kids used to play with)
2. Australia, NZ (?):- to 'dob' someone in means to squeal on someone, to expose them, tell on them