In the top half of a double boiler (or you can use a small pot that fits on top of a bigger pot of boiling water), melt 500g plain chocolate, 175g white chocolate and 50g butter until combined and melted through.
Stir in the 350g icing sugar and 250ml Irish cream until mixture is smooth. Stir in nuts. Place mixture in the prepared tin and lay a sheet of cling film over top; press and smooth top down.
In the top half of a double boiler melt remaining chocolate until soft. Remove from heat and with a fork beat in the remaining butter and Irish cream until smooth. Spread topping over cooled fudge with a knife. If a smooth top is important place cling film over the top. Refrigerate until firm, 1 to 2 hours at least. This fudge can be easily frozen.