Preheat the oven to 350 F and prepare a 24 count cupcake pan with liners.
In a large sauce pan over medium, heat Guinness and butter until the butter melts and begins to lightly simmer. Add the cocoa powder and whisk until smooth. Set aside and let cool while working on the next step.
In a mixing bowl, combine flour, sugar, baking soda, and salt. Lightly whisk to combine.
In a larger mixing bowl, or the bowl of a stand mixer, combine eggs, sour cream, and coffee. Beat eggs until blended. Add the slightly cooled beer mixture to the eggs and beat until just combined. With the speed on low, add flour mixture, adding it about 1/3 to 1/2 at a time. Beat until just combined then pour mixture into the cupcake pan, filling the liners about 2/3 to 3/4 full.
Bake until they pass the toothpick test, about 17 minutes. Remove from pan and let cool on rack until completely cooled, about 1 hour.
Jameson Whiskey Ganache
Place the chocolate in a heat proof bowl. Chop it if the chips are too big, the smaller the pieces are, the faster this will go.
Heat the cream until simmering, but be careful not to boil, then pour over the chocolate. Let sit one minute, then stir until smooth. If the chocolate doesn't all melt, gently warm it over a double boiler. Stir in the whiskey.
Cover and refrigerate while the cupcakes continue cooling.
Bailey's Buttercream Frosting
Once the cupcakes have cooled, scoop out about 1-2 tbs of the inside of the cupcake using something like a small knife, a small ice cream scoop, or a 1 inch cookie cutter. Spoon 1-2 tbs ganache into the cupcake center.
Using the whisk attachment of an electric or stand mixer, whip the butter on medium high for 5 minutes, scraping the sides as needed. Reduce the speed to medium-low, then begin slowly adding the powdered sugar, scraping the sides as needed. Once the 4 cups have been incorporated, increase the speed to medium high and add the Bailey's, whipping for 2 to 3 minutes until light and fluffy. If it isn't setting, slowly incorporate more powdered sugar to stiffen the texture.
Frost the cupcakes with an offset spatula or a piping bag. You can even do that trick of putting the frosting in a zip locked back and cutting off the corner to make a DIY piping bag.