Combine sugar and water in a saucepan over medium-high heat. Cook, stirring constantly, 5 minutes or until sugar dissolves. Remove from heat, and cool completely; set aside.
Stir together tequila, triple sec, lime zest, lime juice, and reserved simple syrup.
Pour mixture into the container of an electric ice-cream maker, and freeze according to manufacturer's instructions. Or, pour mixture into a 13- x 9-inch baking dish, and freeze 3 hours, stirring every 30 minutes to break up large ice crystals. Garnish, if desired.