Combine the water and sugar in a small pot over medium heat. Bring to a slow boil, then reduce the heat to a simmer and cook for 5 minutes. Remove from the heat and let cool for 10 minutes. Stir in the limoncello and lemon juice, then pour the mixture into an 8-by-8-inch freezer-safe dish. Freeze, scraping and mixing with a fork every 30 minutes, until all the liquid is frozen, about 2 hours.
Serve in small tumblers, garnished with mint sprigs.