In a cocktail shaker or mixing glass, place the fresh mint leaves and the thin cucumber slices. Add the fresh lime juice and simple syrup. Gently muddle 6–8 times to release mint oils and cucumber flavor — avoid over-muddling the mint to prevent bitterness.

Build and chill

Add the Irish whiskey and green chartreuse to the shaker with the muddled mixture. Add a handful of cubed ice (about 1 cup). Secure the shaker and shake vigorously for 10–12 seconds until the outside of the shaker feels cold.

Strain

If you prefer a bright, clear cocktail: double-strain the mixture through a fine mesh strainer into a chilled coupe or rocks glass. If you prefer a slightly textured drink with more aromatics, strain once into a rocks glass over 1/2 cup crushed ice.

Finish

Top the drink with about 1 oz club soda for a light effervescence (optional). Gently stir once to integrate.

Garnish and serve

Garnish with a lime wheel or a cucumber ribbon and a small sprig of mint. Serve immediately.