In the top half of a double boiler melt the 3 cups semisweet chocolate chips, 1 cup white chocolate chips and 1/4 cup butter until soft enough to stir.
Stir in the confectioner's sugar and Irish cream until mixture is smooth. Stir in nuts. Place mixture in the prepared pan and lay a sheet of plastic wrap over top; press and smooth top down.
In the top half of a double boiler melt remaining chocolates until soft. Remove from heat and with a fork beat in the butter and Irish cream until smooth. Spread topping over cooled fudge with a knife. If a smooth top is important place plastic wrap over the top. Refrigerate until firm, 1 to 2 hours at least. This fudge can be easily frozen.