I agree with Big Mick and Woodsie. If the dialect is on the mark, then more credit to the non-Irish/Scottish singer. Since there are so many different regional dialects in both countries, if a person that isn't from either place goes to the trouble of being aware of the region that the song is from, and then does their best to get the dialect right, then I'd say good on them. Even if they're not so slick at it, they don't intend any offense, and don't deserve to be called wankers, unless of course they really are wankers.

I would rather hear non-Irish singers sing in an Irish accent than non-Scottish singers murdering the Scots dialects. I'm Scots/Irish by blood, though not actually from either. I'm just a Yank. But when I sing a proper Irish song like Grace, Fields of Athenry, or Red is the Rose, I sing it in what I've been told is a pretty damn good "Oirish" accent by those qualified to judge (Irish folk from Ireland). I can readily tell the difference between an Ulster accent and a Dublin accent, and I find both much easier to emulate than any of the Scots dialects. I absolutely cringe when I hear some ignorant twit trying to sound Scottish. It embarrasses me. Mel Gibson did a decent Scots accent in Braveheart, or at least I thought so. And Richard Gere was right on as Declan Joseph Mulqueen in the remake of The Jackal. I'd say it's more difficult to speak with an unaccustomed accent than it is to sing with one.