SHRIMP: Soak 8 (4") bamboo skewers in water for 10 minutes to prevent burning. Preheat grill to medium heat. Combine all ingredients except shrimp and skewers in a small mixing bowl. Line shrimp up in vertical rows of five, all facing the same direction on a flat surface. Thread two wooden skewers, one at the head-end and one at the tail-end, through each row of five shrimp so you end up with a washboard-looking rectangle of skewered shrimp. Place shrimp on a platter and brush all over with garlic mixture. Place shrimp skewers on hot grill and cook 1-2 minutes per side.
MANGO RELISH: Heat olive oil in a small skillet over medium heat. Saute onion, garlic and chile pepper until soft and fragrant. Turn off heat and stir in sugar and diced mango. Season to taste with salt and pepper.