The drink served is called the "Merry Merry Mule". Watcher gives the recipe:
Ingredients:
Mule Mug, Gin, Mint, Lime, Cranberry Juice, Ginger Beer, Cranberries, Rosemary
1) Muddle 8-10 Mint Leaves
2) Add 2 oz Gin, 1/2 oz Lime Juice, 1 oz Cranberry Juice.
3) Shake and strain over Mule Mug (filled with ice). Top with Ginger Beer
4) Garnish Cranberries and Rosemary