1Soak dried mushrooms in boiling-hot water in a bowl until softened, about 20 minutes. Drain in a paper-towel-lined sieve set over a bowl and reserve soaking liquid. Rinse mushrooms, pat dry, and finely chop.
2Bring soaked mushrooms, soaking liquid, ribs, salt, pepper, and remaining 8 cups water to a boil in a 6- to 8-quart pot, then reduce heat and simmer, partially covered, skimming foam, until meat is just tender, about 1 hour. Transfer ribs with a slotted spoon to a cutting board to cool, reserving broth.
3While meat simmers, cook onion, carrots, celery, and creminis in oil in a 12-inch heavy skillet over moderate heat, stirring occasionally, until well browned, 15 to 20 minutes. Stir in barley and cook, stirring, 1 minute.
4Discard bones, fat, and gristle from meat, then cut meat into 1/2-inch pieces and add to broth along with barley mixture. Simmer, uncovered, until barley is tender, about 40 minutes. Skim fat from surface, then stir in dill and salt to taste.