It's a song having a pop at your footballing rivals, not a very tasteful one I admit, but not grossly offensive in it's insinuations outside of the stadium.
The Billy Boys on the other hand is celebrating triumphalism over a minority of the population, who may or may not have anything to do with Celtic. As mentioned elsewhere swap "Fenian" in the lyrics with "Nigger" or "Paki" and does it seem acceptable?