The main thing is intent. The intent of the person telling it - and the understanding of the way the people listening are going to take it.

What I mean is, if you tell a joke to a racist, and you know that he'd enjoying it as a racist, that's a racist joke even if that isn't how you you meant it it in the first place. And if you tell another like that to follow up, and you know you'll get a laugh, and that's why you are telling it, you are being racist.

You just can't do it by rules and numbers and checklists of what'd offensive and what's not. Otherwise you end up in ridiculous situations. If someone is setting out to insult another person because of their colour or their religion or gender or whatever, they can do it saying the most seemingly harmless things.

So you can end up with suggestions that you shouldn't say "white" coffee, because it is quite possible to say that in a way which is insulting. Which is perfectly true. But it's the intent, not the words that is the problem."Would you like a banana?" can be a insult, but trying to make people call them "plantains" wouldn't do anybody any good.

And the anti-Irish anti-Catholic jokes can be used in an intentionally insulting sectarian way. Maybe your friend was wrong to take it as meant that way. Maybe he wasn't. There's a lot of sectarianism around. (And even the Campbell jokes could be used intentionally as a provocation, a bit of coat trailing, or be taken as meant that way.)

The trouble with going by intent is that there's a subjective element, and you can be mistaken on both sides. But there's no substitute for it.