First get your cream lightly whipped to a thick consistency, but not whipped to fluffiness. Then fill a glass - preferably one with a handle as things are going to get hot! - with hot water to preheat it, then empty. Pour the hot coffee into warmed glass until it is about 3/4 full. Add the brown sugar and stir until completely dissolved. Add the Irish whiskey. Add the whipped cream by pouring gently over back of spoon so it just floats on top. Be generous with the cream: you want a nice, thick layer so you can sip the coffee through the cream. Being generous with the whiskey is not a bad idea either. Serve immediately.