When you come to pronounce Mairi in the song, you don't have much choice about how long to hold the "ah" sound it - the song rhythm tells you that, and also puts the stress on the first syllable.

The only thing left for the Sassenach to decide is which "a" to choose (as in "marry" or as in "far"). In fact, Scots speakers make little or no distinction between these two sounds, so just aim for a general middling-open "ah".

Marje