Reserve and refrigerate ½ cup whipped topping for garnish. In a large bowl, whisk together root beer, milk and pudding mix for 2 minutes. Fold in half of the whipped topping. Spread into graham cracker crust.
Spread remaining whipped topping over pie. Freeze at least 8 hours or overnight.
Dollop reserved ½ cup whipped topping over each serving. If desired, top each serving with a maraschino cherry.
