Set aside and refrigerate 1/2 cup whipped topping for garnish. In a large bowl, whisk the root beer, milk and pudding mix for two minutes. Fold in half the remaining whipped topping. Spread into graham cracker crust.
Spread remaining whipped topping over pie. Freeze for at least eight hours or overnight.
Dollop reserved whipped topping over each serving; top with a maraschino cherry if desired.