Irish whiskey is often matured in oloroso sherry seasoned casks so it's no surprise that whiskey and sherry blend harmoniously in this cocktail, with lemon...
I can confirm for any curious/desperate cocktail enthusiasts in the UK that this works with 1 tbsp redcurrant jelly if you stir it in with the lemon juice to dissolve. Probably a touch drier due to the lower sugar content than a syrup would have, but a perfectly tasty drink!