Place the chicken and 1 cup salad dressing in a bowl. Cover, and marinate at least 3 hours in the refrigerator.
Preheat the grill for high heat.
Lightly oil the grill grate. Discard dressing used for marinating, and grill chicken 7 minutes per side, or until juices run clear. Cool and shred.
In a large bowl, mix the cooked chicken, spinach, feta cheese, sun-dried tomatoes, and remaining dressing.
Brush the focaccia bread with olive oil, and cook 1 minute per side on the prepared grill, or until lightly toasted. Place portions of the chicken mixture on the toasted focaccia to serve.