Line a 9x13-inch cake pan with parchment paper or foil, leaving 2 inches on each side for easy lifting after the fudge sets. Set the pan aside.
In a medium saucepan, combine the sugar with the butter and milk. Place on the stovetop on medium heat, until it begins to boil, then stir while it boils for five minutes. It will become bubbly and very hot. Be sure to stir continuously so the mixture doesn’t stick to the saucepan.
Remove the pan from the stove, and add in the chocolate chips and marshmallows. Stir the chocolate until all of the chips and marshmallows are melted. Stir in the Irish cream, (and nuts if you are using them), and continue stirring until the cream mixes into the fudge.
Pour the fudge mixture into the prepared pan, and spread it evenly. Allow the mixture to cool until it is set.
Cut into one-inch squares and serve.
Notes
To prevent the fudge from drying out, keep it covered with saran wrap or in a sealed container.
If you don’t have marshmallows on hand, a 7 oz tub of marshmallow creme can be substituted.
Be sure to use evaporated milk and not sweetened condensed milk.