Pour 4 cups root beer into a 13-by-9-by-2-inch baking pan; freeze until set, about 4 hours or overnight.
2.
Meanwhile, slowly boil remaining 4 cups root beer in a large saucepan until reduced to 1/2 cup, about 1 hour. Let cool, then chill.
3.
Using tines of a fork, scrape frozen root beer in baking pan, making icy flakes, then mix gently in pan to blend. Spoon 3/4 cup of the frozen root beer granita into each of 6 clear glasses or dessert cups. Top granita with a small scoop of ice cream. Drizzle each with 4 teaspoons root beer syrup and serve.
