For those in the know, the Miss MacLennane of the song isn't a beautiful girl, but a legendary sweet stout. It doesn't appear to be commercially available, although the recipe can be found on the internet, for all the home brewers out there.

Sally MacLennane Stout

Ingredients
1.8 kg Best Cellar Stout Kit (or Irish malts if you can get 'em)
0.5 kg dark DME
500 g flaked barley
500 g roasted barley
250 g crystal
about 10 g Cascade (boiling)
40 g Goldings (boiling)
20 g Goldings (finish)
Wyeast 1084 Irish Ale

This recipe was originally found at www.poguetry.com
And a review of the success of this recipe can be found at hbd.org/brewery/cm3/recs/05_106.html