Preheat a grill to medium high. Stack 2 large sheets of heavy-duty foil; spread the potatoes and onion wedges in the center. Fold up the sides to form a bowl shape. Dot the vegetables with2 tablespoons butter and sprinkle with 1/2 teaspoon salt; pour in about two-thirds of the beer. Lay another large sheet of foil on top; crimp the edges to seal. Set the foil packet on the grill; cover and cook until the potatoes are fork-tender, 20 to 25 minutes. Meanwhile, stack 2 more large sheets of foil and spread the shrimp in the center. Fold up the sides to form a bowl shape. Dot the shrimp with the remaining 1 tablespoon butter, sprinkle with the Old Bay and pour in the remaining beer. Top with another sheet of foil; crimp the edges to seal. When the vegetables are almost done, add the shrimp foil packet to the grill; cover and cook until the shrimp are pink, 6 to 8 minutes. Sprinkle the vegetables with the parsley, and salt to taste. Serve with the shrimp and the cooking liquid from the packets.