Preheat oven as directed on the cake mix box. Line 20-24 cupcake cups with liners.
In a mixing bowl, combine cake mix, oil and eggs and mix until combined. Slowly pour in Guinness and mix on low for 30 seconds. Turn mixer speed to medium and mix for one minute. Divide the batter equally among the cupcake cups and bake according to the package directions. Let cool completely.
In a small saucepan, melt butter over medium heat. Add chocolate and stir until melted and smooth. Stir in cream and whiskey. Let cool.
Beat butter in mixer until smooth and fluffy. In 2-3 Tbsp increments, add powdered sugar letting each prior fully incorporate into the frosting before adding more. After 2 1/2 cups of sugar, beat in Irish cream. Add additional powdered sugar if needed for desired consistency.
Using a small paring knife, cut a small piece out of the top of each cupcake. Fill with a small amount of ganache and replace the cupcake piece.
Frost cupcakes as usual.
If you want to make little chocolate pieces to go on top, place a piece of parchment paper on top of a wine bottle. Melt chocolate chips in a plastic bottle and squeeze onto top of the parchment paper. Let set and break into pieces.