Place the lamb in a heavy 4-5 quart casserole and add the water. Bring to a boil over high heat, meanwhile skimming off the foam and scum as they rise to the surface.
Add the barley, salt, pepper, and reduce heat to low, and simmer partially covered for 1 hour. Add the carrots, turnips, onions, leeks, and celery, partially cover again, and cook for 1 hour more.
With a slotted spoon, transfer the lamb to a plate and pull or cut the meat away from the bones. Discard the bones, fat and gristle, and cut the meat into 1/2 inch cubes. Return the meat to the soup and simmer for 2 or 3 minutes to heat it through.
Taste for seasoning. Sprinkle with parsley before serving.