Place the barley and 1 1/2 cups of the water in a large saucepan or a Dutch oven. Bring to a boil, cover, and simmer until the barley is tender (20 to 30 minutes).
Meanwhile, melt the margarine in a skillet. Add the onions and saute' for about 5 minutes over medium heat. Add garlic, mushrooms, and salt. Cover and cook, stirring occasionally, until everything is very tender - about 10 to 12 minutes. Stir in soy sauce and sherry.
Add the saute' with all its liquid to the cooked barley, along with the remaining 5 cups of water. Grind in a generous amount of black pepper, and simmer, partially covered, another 20 minutes over very low heat. Taste to correct seasonings, and serve.