Yes, she is too young to understand discipline, but she is not too young to understand simple rules: We eat in restaurants. We scream outside. Dr. Karp, in The Happiest Toddler on the Block, suggests that toddlers want you to understand them. If you can repeat back what you're "hearing" them say--in their "language"--they do not need to scream or throw a tantrum. Basically, it's like a fast-food worker reading back your order to you to be sure you understand each other.
Of course, he adds that meltdowns and screaming happen when they're hungry, bored, or tired, so you have to make sure they're fed, stimulated, and rested.
We've been doing this with our daughter (18 m.o.) for several months, and it really works with her. We give her crackers or a snack to take the edge off in restaurants (I get cranky, too, when I can smell and see the food but can't have any yet!) then say, "Iris wants to eat! Want to eat NOW! NOW, NOW, NOW!!" or whatever I feel she's trying to say. She stops, looks at me like, "hey, you GET me!!" and nods her head yes. When I have her attention and she's calmed down, I say, "Iris wants to eat, but we don't have our food yet. You can have some crackers if you'd like, or we can go outside until you're calm. Restaurants are for eating, and outside is for screaming."
Hope that helps!