01 - Fill a cocktail shaker with ice cubes to chill the vessel.
02 - Pour Irish whiskey, green crème de menthe, lemon juice, and simple syrup into the shaker.
03 - Shake vigorously for 15 seconds until well chilled and properly diluted.
04 - Strain into a chilled cocktail glass.
05 - Garnish with a lemon twist or fresh mint sprig and serve immediately.