Add ginger beer, cranberry juice, rum, and the juice of a squeezed ½ lime in a cocktail shaker filled with ice.
Stir or very gently shake, then pour into the copper mug.
Top with cranberries and garnish with a lime wheel and serve.
Notes
Do not overshake the mixture as there is ginger beer in the drink. Gently shake for a second or two or stir. You can also omit the ginger beer in step 2 and pour over just before serving.