To a mug, add 5 ounces coffee. Stir in white and brown sugar until dissolved. Stir in Irish whiskey and top with whipped cream.
Notes
Irish whiskey: A simple blended Irish whiskey like Jameson or Bushmills works great. If you don't have or don't enjoy Irish whiskey, try the same base cocktail recipe with bourbon (for a Kentucky Coffee) or Bailey's (Irish Cream Coffee) in place of the Irish whiskey.
Coffee: Use piping-hot, freshly-brewed coffee. Espresso, warmed-up cold brew, French press, pour-over, or even percolator-brewed will do.
Sugar: Aim for equal amounts of white and brown sugar; the warm and slightly smoky molasses in the brown sugar complements the whiskey well.
Whipped cream: It's worth the effort (and easy) to make homemade whipped cream. For best results, start with chilled bowl, chilled whisk, and cold heavy cream.
Yield: Each recipe makes 1 drink. Rather than making this punch-style in a big batch, I recommend assembling as many individual glasses of 1-serving Irish Coffees as you need to share with your guest list.
Preheat your glasses: The alcohol in the drink can cool things off, so if you have time, preheat the glasses with a splash of hot water (then dump it out) to prime them for the hot coffee.