Place shanks in slow cooker and add half of carrots, celery and onion. Pour in broth, cover and cook on low until tender, about 8 hours. Remove shanks to a platter, cover with foil and refrigerate. Strain liquid into a large bowl. Cover and refrigerate for 5 hours or overnight. When liquid has chilled, remove solid layer of fat that forms on top. Remove meat from shanks, discarding all fat and gristle. Cut lamb into small pieces.
Pour defatted liquid into a large pot. Add remaining carrots, celery and onion. Stir in lamb and barley. Bring to a boil over high heat. Reduce heat to medium-high and cook until vegetables and barley are tender, about 30 minutes. Season with salt and pepper. Sprinkle with parsley, if desired, and serve hot.