Ko Adang's restaurant is located near the park accommodation, and is open daily 07:30-15:00 and 17:30-21:00. They serve basic Western breakfasts and sandwiches, and a range of quality Thai food.
We tried a green curry that was authentic and flavourful, and the portion was large. Prices are also far cheaper than on Ko Lipe (or Ko Tarutao for that matter, which we still can't figure out).
Fried rice and stir-fries run just 55 baht for veggies, 65 baht for chicken, pork or beef, and 75 baht for seafood. In the 100 baht-plus range they also have tasty and authentic Thai salads and grilled fish when available for 180 baht.
They do not serve beer, however, and alcohol is no longer permitted on Ko Adang.