For years, I was certain that the Irish Rovers were faking. Oh, well.
Personally, I'd rather everybody just used their own accents. Well, most of the time--I can't picture something like "Danny Boy" sung with an East Texas accent. But, as far as I'm concerned, if you identify with a song that well, how you talk/sing won't matter.
I'm sure there are accent prejudices in Europe as well as in the US. My mother worked really hard to lose her Southern New Jersey accent, which got her teased in college and was a sore point when she started dating my dad, who comes from a family of upwardly-mobile, conspicuously accent-less midwesterners (the rest of her family still sound like B-movie gangsters). I don't have much of a natural accent and I'm a *terrible* mimic; I wouldn't think of intentionally trying to adopt an accent for fear I might offend somebody ;).