Reduce the Guinness. Boil the Guinness for at least 30 minutes to get a concentrated flavor that comes through in the cake.
Use cold cream cheese & heavy cream. In order to create a sturdy whipped cream cheese frosting, pay close attention to the temperature of the ingredients. Cold cream cheese will ensure it's thick enough, while cold heavy cream will whip up and give the frosting enough structure.
Cream the cream cheese & sugar thoroughly. Cream the two until completely smooth before adding the heavy cream to avoid over-mixing and risk the frosting splitting!