Bloom the gelatin: Sprinkle 2 tbsp gelatin over ½ cup cold water. Let sit 3–5 minutes until spongy.
Cool slightly: Let the mixture sit 2–3 minutes so the alcohol doesn’t cook off.
Add remaining ingredients: Stir in Irish whiskey, lemon juice, and simple syrup.
Fill lemon halves: Place hollowed lemon halves in a muffin tin so they sit flat. Pour mixture into each.
Chill: Refrigerate 3–4 hours until firm.
Slice: Cut each lemon half into 3–4 wedges.