Line 9-inch square pan with foil. In large bowl, combine popcorn and all but 2 tablespoons graham cracker pieces.
In large glass bowl, microwave marshmallow crème and butter on high 1 minute. Stir until butter is melted. Stir in lime peel and lime juice.
Pour marshmallow mixture over popcorn, mixing thoroughly. Using damp hand, firmly press mixture into prepared pan. Sprinkle with reserved graham cracker pieces. Refrigerate 2 hours until firm.
Lift foil from pan. Break popcorn mixture into clusters.