I like some English stuff, like The Levellers and The Men They Couldn't Hang, and there's a lot of good Celtic Punk, like Flogging Molly, The Tossers and The Real McKenzies (little of it, oddly enough, from anywhere near the Celtic Nations). I haven't listened to much in the way of American non-Celtic Folk Punk, but I've gotten hold of a couple of Against Me! albums, and, if they're any indication, I probably should.
Folk is a method of transmission and a mode of composition more than anything else. I wouldn't really call a band 'folk punk' unless they were using trad tunes and words, doing old wobbly songs or something maybe, or at the very least engaging with, commenting on and building on the folk tradition. Not just they wanted to play punk but couldn't afford amplifiers.
Even if you're using "folk" to mean "ethnic traditional music", I'd say it was more about musical style, than about direct use of traditional sources. A lot of Traditional-style folk songs are relatively recent themselves, such as the famous "The Fields of Athenry", which was only written in the 1970s.
