In a food processor, combine the butter with the parsley, lime zest, garlic and crushed red pepper. Season with 1 teaspoon salt and 1/2 teaspoon black pepper and pulse to blend. Scrape the butter onto a sheet of plastic wrap and roll into a log. Refrigerate until chilled.
Light a grill. Generously season the skirt steak with salt and pepper and grill over a hot fire until the meat is browned on the outside and medium rare within, about 3 minutes per side. Transfer to a carving board and let rest for 5 minutes. Thinly slice the steak across the grain. Serve right away, passing the parsley-lime butter at the table.