Preheat oven to 400 degree F. In a small bowl, combine dressing and horseradish. Using half of the butter, lightly brush half of the bread on one side. Place on a 15x10x1-inch pan, buttered side down.
Spread the dressing mixture evenly on each slice of the bread. Top with corned beef and then cheese. Brush remaining slices of bread with remaining butter, and place atop cheese, butter side up.
Bake for 8 minutes. Turn, and press lightly with a spatula. Return to oven and bake 5 minutes more or until golden brown. Let cool for 5 minutes; cut in half diagonally. Serve with small bowls of mustards for dipping, if desired. Makes 16 cocktail sandwiches (8 servings).