Fill a pint glass half-full with the oatmeal stout and let it settle.
Combine the whiskey, half-and-half, Simple Syrup, creme de menthe and cinnamon in a cocktail shaker. Add ice, shake vigorously and double-strain into a shot glass.
Top off the oatmeal stout pint and let settle.
Serve the shot and stout together. Shoot or sip the Irish creme and drink or sip the oatmeal stout.
Combine the sugar and 1 cup water in a small saucepan and bring to a boil. Cook, stirring, until the sugar dissolves. Remove from the heat and let cool completely. Keep covered in the refrigerator up to 1 month.