Combine first 4 ingredients in a saucepan. Bring to a boil over medium heat. Boil 5 minutes. Remove from heat, and let cool completely. Remove and discard rosemary.
Peel and section grapefruit over a serving bowl to catch juice. Add grapefruit to bowl. Pour rosemary syrup over fruit in bowl. Add cherries. Cover and chill until ready to serve. Garnish, if desired.