I just want to add to the discussion that the main reason I started learning Irish was because I fell in love with sean-nós singing. Aside from the pronunciation issue (Which, as others have said, isn't a big issue if the singer takes the time to learn to do it properly), I really felt that I couldn't do the songs justice without understanding them on a very deep level (not just in translation).
I do think there's a difference between this kind of singing and classical singing (I do both). In classical music, a great deal of the emotion is actually written into the music...it's nearly impossible to sing it with the "wrong" interpretation, if you follow the music, the dynamics, and the editorial markings. Take the Dies Irae from Mozart's Requiem (which I've sung on several occasions)...the intensity in that piece doesn't come so much from the singers' understanding of the words as from the way Mozart wrote the music and from the way the conductor interprets it.
(BTW, if anyone out there is unfamiliar with Mozart's Requiem, here's the Dies Irae. I always keep it in my CD changer so, when I pull up next to someone blasting rap from his speakers, I can cue up Dies Irae, crank up the speakers, and go into my mad conductor routine.
http://www.youtube.com/watch?v=j1C-GXQ1LdY)
With sean-nós songs, you can't really tell all that much from the basic tune...or even, really, from hearing a really good singer (though you do need to hear good singers if you're going to learn the style). You can get the pronunciation exactly right, imitate another singer's style exactly, and still not do the song justice.
You don't have to be a native speaker, but I think you do need to have some understanding of Irish to really do it. Ideally, you'll also know the back story behind the song, and have some familiarity with the culture. From a musical standpoint, I'd rather hear someone whose pronunciation isn't quite perfect, but who clearly understands the song, than someone who can replicate the sounds of Irish perfectly but who doesn't understand the words he or she is singing.
Of course, I'd like them to work on that pronunciation too! But as a musician, nothing bothers me more than someone singing or playing a traditional tune who doesn't really "get" it. It's possible to do so, in some cases, without speaking the language, but it's a lot harder.
Just my two cents.
Redwolf