While it has no Irish ties and isn't naturally green, the recipe Spencer Blake uses for Orange Julius can fit right in with your green feast (and it's great when it's boring ol' orange, too).
1/2 - 12 oz. can frozen orange juice concentrate
1/2 cup sugar
1 cup green milk (or white is fine, too)
1 cup water
1 t vanilla
1 tray ice cubes (about 14)
Put all ingredients in blender. Use the 'ice crush' setting until ice is in small flecks (still large enough to add some crunch)