In the bowl of a mixer beat butter and cream cheese until smooth.
Mix the powdered sugar with the Irish cream until it makes a slurry.
Pour the slurry into the butter mixture on medium speed and mix for a minute, scrape the bowl and continue to mix for 3 minutes so the frosting will be soft and fluffy.
Spread the frosting over a cooled cake
Instructions for the shamrock are in the post: Cut a shamrock out of paper, waxed paper, or parchment paper, place it on top of the frosting, and add a dusting of unsweetened cocoa in a fine strainer to cover the cake.
Place cake in fridge and remove 30 minutes before serving.