I almost think we could be more productive discussing phrasing, diction, storytelling, relaxing and having some fun with the material rather than vocal quality, per se. Most people I have heard who used altered voices for different material sound contrived and false anyway.

I know I'm not a Scot, nor an Irishman, but I have enjoyed doing many songs of those lands. I just try to deliver the essense of the song, have a little fun with it and not worry too much about sounding "authentic."