Oh yes, it's certainly bound to be a very small amount. They're still there though and you're still more likely to support them through that charity than through, say, donating to the local homeless shelter, or the local GAA club, or by giving money to a Derry child fighting cancer, or by paying for the funeral of a local fan, all of which McClean has actually done. As I said, it's much easier to avoid supporting war veterans when you don't support a charity that specifically aids war veterans.
As for the bold, as far as I can see that's utter sophistry. The UK is his country and taxes are collected on that basis, no? I mean if taxes collected in NI stay in NI and in no way fund the UK's army then feel free to correct me, I'll happily admit I'm wrong. If not then he's forced to pay taxes to that same government regardless, unless you expect him to refuse to ever work, live or pay taxes in his own country?