Bring water to a boil, add salt and barley, stir, and cook covered for 30 minutes, or until all water is absorbed. Let sit covered a few minutes, then put into plastic mixing bowl and let cool.
While barley is cooking, wash mint and finely chop. (I pull the mint from the stems and wash in a salad spinner, spin very dry, and then chop in a food processor.) Chop tomatoes and let drain in colander for at least 15 minutes. Chop olives and onion.
Combine dressing ingredients in small glass jar and shake well.
When barley is cooled, combine with mint, tomato, red onion, capers, and olives. Pour dressing over and stir a few times to combine. Serve cold or at room temperature.