Preheat the oven to 350 and prepare 12 standard cupcake cups by oiling them or using paper cup liners.
For the cupcakes: beat together the beer, oil, egg and sugar until smooth. Sift the dry ingredients in and combine. It might look lumpy, but that’s okay; I think it’s just beer bubblies.
Divide the batter between the cups, using a scant 1/3 cup of batter for each.
Bake for 22 minutes or so, until they pass the toothpick test. They might be a little flat-topped but I’m not sure how to get around that. It’s okay.
Remove them from the pan and cool on racks.
For the frosting: Beat the soft butter on medium speed until creamy and fluffy. Add the sugar a few tablespoons at a time. When you’ve added about a cup and it’s looking really stiff, add a couple tablespoons of the liqueur and beat it some more. Add some more sugar and go until you like the consistency, adjusting it with more liqueur or sugar. You should end up with about a cup and a half of icing.
Frost the cupcakes and decorate them with little chocolate dealies or sprinkles or some cute-ass green shamrock sprinkles.
I like a light frosting on mine, so this frosting recipe doesn’t make a whole lot. Double it if you’re a big frosting person.