Lemongrass is one of the most prevalent herbs in Thai cooking and can be found in Asian markets. The stalks impart their flavor when used as skewers.
Separate cilantro leaves from stems; set aside leaves (you should have 1 cup). Roughly chop stems; combine with ginger, garlic, chopped lemongrass, 1/2 cup oil, and coriander in a blender or food processor, and process to form a paste.
Place shrimp in a large, shallow bowl; add the paste, tossing to coat evenly. Cover with plastic wrap, and let marinate in the refrigerator 1 hour.
Make cilantro dressing: In a small bowl, whisk together reserved cilantro leaves, lime juice, remaining 1/2 cup oil, sugar, and salt; set aside.
Heat grill to medium. Remove shrimp from refrigerator, and thread one on each skewer; season with salt and pepper. Grill, turning once, until shrimp are opaque, about 3 minutes per side. Transfer to a platter. Serve hot with dressing.