There are a group of right wing city fans who jump on the celtic-rangers thing to basically be anti-Irish. And yes it is racist/xenophobic to call people fenian bastards. Didn't have any problem with any celtic fans in Glasgow but did get abuse from some middle aged City fans for not going in for the anti Irish bullshit (I'm not even Irish btw)
The anti-English sentiment bubbling in Scotland and Ireland absolutely dwarves that you've mentioned by a huge amount.