soccer? if you call it soccer, you're not British.
I'm born & bred Leeds, support Leeds Utd but couldn't give a rat's arse about England, the national team representing the country of my birth. Does that make me not-British? Or not-English anyway?
no, it's an accident of birth
and as far as I remember, my passport says "United Kingdom of Great Britain & Northern Ireland". So if your friend has a UK passport, I'd say that makes him (a) Irish and (b) a UK citizen; if he has an Irish passport then he'd be just Irish.
To be British, would one have to be English, Welsh or Scottish, under that definition?