The main restaurant on Ko Tarutao is located next to the Visitor's Centre at Ao Pante Malaka. It's open 08:00-19:00. Be sure to get there early in the evening because if you show up at 20:00 be prepared to beg someone to open the shop and fix you some Mama noodles.
The food is a bit pricey (it's not like they have anyone to compete with), but portions are large and overall the food is of good quality. On the cheaper side, a 100 baht order of seafood fried rice and a heaping 80 baht plate of pad see ew fried noodles was enough to fill two of us up after a long day of biking and hiking. In the 200+ baht range they offer grilled fish if available.
The menu is translated into basic English, but if you know Thai food, chances are they'll fill your request. The restaurant also offers basic Western sandwiches, fixed price breakfast combos and a selection of cold drinks, including beer. There's also a restaurant at Ao Molae, although it doesn't open until December. Expect similar prices and quality there.