Mix the onion, garlic, oil, and fish sauce in a bowl large enough to hold the shrimp.
Add the shrimp, toss to coat evenly, and marinate, covered, at room temperature for 30 minutes.
Soak the wooden skewers in water for 20-30 minutes.
Prepare grill or preheat broiler.
To prepare the dip, mix together the lime juice, pepper, salt, fish sauce, and sugar, in a small bowl, mixing well; adjust seasonings to taste (add a little water or sugar if dip seems too tart).
Thread shrimp onto wooden skewers; lightly coat each finished skewer with oil or cooking spray, and place on grill over medium coals or heat (or place on pan under broiler), and cook approximately 5 minutes, turning once, or until shrimps are pink, firm, and slightly charred in spots.
Serve shrimp with the dipping sauce.
A good accompaniment would be steamed rice, or an Asian noodle dish.