Add the water, cranberries, sugar, cinnamon stick, rosemary, salt and lemon zest to a medium saucepan. Bring to a boil over medium heat. Lower the heat and simmer until slightly reduced in volume and thickened to a syrup. Remove from the heat and strain through a fine mesh sieve into a bowl. Discard the solids. Refrigerate until cool, about 30 minutes.
To make a drink:
Put a handful of ice in a drink shaker along with 1 1/2 ounces of whiskey and 1 tablespoon of the cranberry syrup. Shake gently a few times and strain into a glass. Repeat for the remaining drinks and serve.