Combine the crust ingredients in a large bowl. Mix well making sure that all the crumbs are well coated with butter. Press evenly into the bottom of an unlined 9-inch springform pan. Tamp the crumbs down tight with a flat-bottomed tumbler. Refrigerate until firm.
Whip the cream in the bowl of an electric mixer and gradually add the granulated sugar. Add the vanilla. Whip until stiff peaks form; set aside.
Filling layer one
Microwave 25 (178g) of the large marshmallows in a large heatproof bowl for 45 seconds or until they have puffed up considerably. Remove from the microwave and stir until the marshmallows have melted and completely lost their original shape. Stir in 1/4 cup Irish cream liqueur. Add the 4 ounces of melted semisweet chocolate and fold together. Add 2 cups (1/3 portion) of the whipped heavy cream and fold through. When the mixture is consistently cocoa-colored, pour it on top of the crust in the springform pan. Place in the freezer and chill for 30 minutes.
Filling layer two
Microwave the remaining 25 (178g) large marshmallows in a large heatproof bowl for 45 seconds or until they have puffed up considerably. Remove from the microwave and stir until the marshmallows have melted and completely lost their original shape. Stir in 1/4 cup Irish cream liqueur. Add 2 cups (1/3 portion) of the whipped heavy cream and fold through. When the mixture is consistently light tan colored, pour it on top of the chocolate layer in the springform pan. Place in the freezer to chill for 30 minutes.
Final layer
Spoon the remaining 1/3 portion of whipped cream on top of the Irish cream layer, and swirl with a spatula. Refrigerate until the pie is well set, about 1 1/2 hours. The pie should be firm and will not jiggle in the center when it is set. Place pretzel twists around the top edge of the pie, if using.
Run a thin knife around the edge of the pan and remove the spring form collar. Slice and serve. Store the pie loosely covered in plastic wrap in the refrigerator.