Hi, Clinton - this song causes some confusion because there is no uniform spelling for "Mary Mack." Try a search for up among the heather and you'll come up with some interesting results, including Lyrics To Mari Mac and Up Among the Heather, and Lyr req: Mari-Mac (Great Big Sea).
That last thread indicates that what you're thing of may be a medley of "Mary Mack" and "Up Among the Heather.
-Joe Offer-