Heat granulated sugar, butter or margarine, and 1/2 cup cocoa powder in a large saucepan over medium heat until butter or margarine melts, stirring constantly. Remove from heat. Add eggs and 1 teaspoon vanilla; beat lightly just until combined.
Stir together flour, baking powder, and baking soda in a bowl; set aside. Stir together milk and 1/4 cup whiskey; stir in coffee crystals. Add flour mixture and milk mixture alternately to the chocolate mixture, beating by hand after each addition. Stir in nuts. Pour into a greased 15x10x1-inch baking pan.
Bake in a 350 degree F oven for 15 to 20 minutes or until a toothpick inserted near the center comes out clean. Cool in pan on a wire rack.
Meanwhile, for the Irish Coffee Glaze, stir together sifted powdered sugar, 2 tablespoons unsweetened cocoa powder, 1 tablespoon Irish whiskey, and 1-1/4 teaspoons vanilla. Stir in 3 to 4 tablespoons brewed coffee until of drizzling consistency.
If desired, sprinkle with additional chopped nuts. Makes 36 brownies.