Having just sung it over to myself I find that I pronounce the Marie in the verse with a longer 'a' than in the chorus. Longer 'a' and more aspirate 'r'.

It is fairly early in my book (no 25) so it was most likely written down around 1969 or 70, but I wonder if I heard bits of it sung by two different people and picked up the pronunciation at different times.

I used to have a very accurate memory for sounds and songs and learned some at one hearing.

I don't think that you have to go back to gaelic to get a song right - if you are singing the rest of the song in your local intonation to try to put in something alien to the area is just plain wrong.