In a large skillet heat oil over medium heat. Cook meat in hot oil until brown on all sides; drain fat.
In a 5- to 6-quart slow cooker combine carrots, celery, tomatoes, broth, barley, onion, the water, and pepper. Add meat to cooker.
Cover and cook on low-heat setting for 7 to 9 hours (do not use high-heat setting) or until lamb pulls easily from bones and barley is tender.
Remove meat from slow cooker; transfer to a serving platter. Skim off fat from vegetable-barley mixture. If desired, stir in vinegar. Serve with lamb. Id desired, top each serving with fresh snipped rosemary.