Bring the barley and water to a boil in a saucepan. Cover, reduce heat to low, and simmer 30 minutes, or until tender.
Meanwhile, heat olive oil in a large saucepan over medium heat, stir in the onions, carrots, and celery; cook and stir until the onion has softened and turned translucent, about 10 minutes. Stir in mushrooms, and cook 5 minutes more.
Pour in beef broth, and bring soup to a simmer over medium-high heat, then reduce heat to medium-low, and continue simmering 15 minutes. Stir in barley, and season with salt and pepper before serving.