Puree the beer, cilantro, onion, garlic, red chili powder, salt, and pepper together in a blender. Prick the skirt steak all over with a fork (this helps tenderize the steak and will allow the marinade to penetrate into the meat), place the steak in a gallon ziptop bag and cover with half of the marinade, seal and work the marinade around the steak making sure it is well covered. Reserve the other half to baste the skirt steak while it cooks. Place in refrigerator for 4-5 hours before cooking.