Fill a pot with lightly salted water and bring to a rolling boil. Once the water is boiling, stir in the shell pasta, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, about 13 minutes. Drain well in a colander set in the sink, and chill by rinsing with cold water.
Combine the cooled pasta, tuna, tomato, cucumber, red bell pepper, green onions, and celery in a salad bowl, and toss lightly to mix. Stir in the ranch dressing and dill, cover, and refrigerate until chilled, at least 1 hour.