You can "use" an accent if you feel the vocal sounds you're making are essential to the sound of the music, that is, if you understand -- instinctively or otherwise -- the genre, its vocabulary, traditions, whatever.

In other words, if you've *internalized* the accent somehow, just like what happens when you "pick up" an accent when you move to a new environment. (I think anyone who's any kind of singer shares that tendency, at least a little bit). If you really have come to know a vocal musical tradition, you've gradually picked up a few of the most basic sounds of an accent or dialect, and also eliminated a few of your own natural pronunciations that sound wrong within that musical vocal context. It's still your voice, mostly at least, and the modifications ideally enhance your opportunity to deliver a nice emphatic, emotionally honest performance.

Can a 21st century urbanite sing auld Celtic balladry? Can a white boy sing the blues? Shit yeah! *IF* they know, love, and serve the music, render it faithfully, and use it to express their own selves, absolutely!

By the by, if you want to work on your Irish intonation, get a copy of anything by John Millington Synge and just sart reading out loud. These would be plays, including "The Playboy of the Western World" and a couple of nice little one-acts. Synge absolutely loved the sounds of English as spoken in the west of Ireland -- the second-language English of the Gaelic-speakers -- and did a great job of reproducing it on the page, back in the 1880s. The prose is so lyrical that it's practically impossible to speak the words without automatically adopting much of the proper inflection and pronunciation. Speaking this stuff is almost like singing.

I don't know nearly as much about the Scotch, but I'm sure there are plenty of devoted readers-aloud of, say, Robert Burns. That's just one example, I'm sure we could go on.

Immerse yourself in the culture of your choice, make a geniune honest effort to listen and understand, and you should feel confident about employing whatever vocal riffs, skills, strategies, etc., you eventually acquire.