Preheat your Irish coffee mug by filling it with warm water, and letting is warm up the glass. Pour out warm water before adding coffee.
Pour 4 ounces of brewed coffee into a warmed mug. Add 1 ounce of Irish whiskey, and add 2 teaspoons brown sugar. Stir.
Gently whip the heavy cream, (double cream) with a whisk. It is important to keep the layers separated in the coffee. This way the person will get the experience of tasting the hot coffee through the cool layer of cream. Use a spoon, flipped over to the backside, and gently pour over the heavy cream.
If desired you can whip the heavy cream until it is firm and dollop it onto the coffee.