To make the dressing: whisk lemon zest, lemon juice, shallots, garlic and pepper in a medium bowl. Gradually whisk in oil.
To make the salad: place barley and water in a large saucepan and bring to the boil over medium-high heat. Reduce heat to low and cook, covered, until barley is tender, 30 to 35 minutes. Transfer to a large bowl and let cool, fluffing with a fork occasionally to prevent sticking.
Cut snow peas in half on the diagonal. Cook peas in a large saucepan of water or steam them until just tender, about 1 to 2 minutes. Drain and cool in iced water.
Add snow peas, capsicum, carrot, parsley and chives to barley. Drizzle dressing over salad and toss to coat well. To make ahead, prepare dressing and salad and refrigerate in separate covered containers for up to 1 day. Toss salad with dressing shortly before serving.