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