In a large mixing bowl, whisk together the cold root beer, cold milk, and root beer concentrate until well combined.
Add the vanilla instant pudding mix to the liquid mixture. Whisk vigorously for 2-3 minutes, or until the pudding begins to thicken.
Gently fold in the remaining Cool Whip (leave ½ cup aside for topping) into the pudding mixture until fully incorporated and smooth.
Pour the filling into the premade graham cracker crust, spreading it evenly with a spatula.
Smooth the top and cover the pie loosely with plastic wrap.
Refrigerate the pie for at least 4 hours, or overnight, to allow it to set completely.
Once the pie is set, spread the reserved ½ cup of Cool Whip over the top of the pie for an extra creamy layer.
Garnish with maraschino cherries if desired.
Slice the pie into wedges and serve chilled.
