Lightly whip the double cream until it is just thickened but still pourable, then set it aside. Pour the hot black coffee into a heatproof glass or mug, add the Irish whiskey and the desired amount of brown sugar, then stir thoroughly to dissolve the sugar. Carefully float the whipped cream over the back of a spoon onto the surface of the coffee, creating a distinct layer. Finish with a light dusting of freshly grated nutmeg atop the cream and serve immediately while hot.