I have to go with Auld Lang Syne.... I love how she lets more of her natural accent out on this one.
do you mean she "tones down" her natural accent when she talks and sings?
I think it's because Auld Lange Syne is a Scottish melody in principal. ( Scottish is Celtic too, after all) It stands to reason that some Irish folks would pick up a Scottish brogue. it differs from Irish, of course, but it would be like a Spanish speaking person adopting a Portugese accent or something like that. Meav has sung a few songs in Scottish Gaelage as well. There are words in A L S that are strictly Scottish in origin. So I think it's the difference between Scottish and Irish brogue that I am noticing here. If that makes any sense.