Irish coffee is a cocktail consisting of hot coffee, Irish whiskey, and sugar (some recipes specify that brown sugar should be used), stirred, and topped with thick cream. The coffee is drunk through the cream. The original recipe explicitly uses cream that has not been whipped, although drinks made with whipped cream are often sold as "Irish coffee".
Whisk cream and sugar together in a medium bowl until stiff peaks form. Divide coffee and whiskey among 4 coffee cups, then top each with 1/2 cup whipped cream. And serve immediately.