Sorry Gotcha, but I don't know any Gaelic speaker who would refer to the Black Watch as the 'speckled/spotted black' . It makes no sense.
The sense of the last two lines of the song which you have posted is very clear, "the lassie lost her virginity in the pulling of the bracken".
Simplicity is the answer, don't look for too much, it's a common theme in folk song.