Something I personally like to do when confronted with the Folkists and Supremacists;
The entirety of my line, as far back as I can trace(about 100 or so. The year 100, that is) is...entirely Germanic. No Celt or Gael blood in these veins, I am easily the most pasty-white person you can possibly imagine. Which, by their reckoning, should make me the authority, because my line is direct from the Allfather himself if the lore is to be believed. And I think the racial dick-measuring is just incredibly stupid.
I do that for two reasons- One, I can use their logic against them. I should be the best example they have(certainly better than most, as they're all mutts and half-breeds compared to myself). Two, I get a certain joy when I see the get really, really upset.
NICE!
The argument I typically use is this: Celtic and Germanic (and Aryan, for that matter), aren't racial terms at all. They're linguistic terms. There's no such thing as "the Germanic race"; Germanic Tribes are so-referred because they spoke Germanic languages; same with the Celts. And, seeing as English is a Germanic language, then everyone who speaks English natively, regardless of genetic ancestry or native nationality, is Germanic.
Unfortunately, I'm not one who takes much joy from people getting upset, particularly at me, so I'd most likely just stay silent if confronted by them in public. It hurts too much. Sometimes the hyper-empathy I get from having Asperger's Syndrome really, really sucks...