Restaurants on the Big Island normally close before 9pm. I wish I knew that before I boarded my inter-island flight from Kauai. Using Yelp's iPhone app, and the help of Twitter friends, I came across Sansei Seafood Restaurant - which is open until 10pm, and 1am on Thursdays and Fridays (I believe Saturdays as well, but check with the restaurant).
Sansei is conveniently located in the Queen's Marketplace in the Waikoloa Beach Resort area - not far from many of the Kona side hotels. I've only tried the sushi and sashimi at the restaurant, and everything we ordered was great, especially since we got everything half off. I don't have much to rave about because I stuck with the usual stuff I order at sushi bars - hamachi, maguro, etc. Add some beer on the side, and you've got yourself a great late night meal.
Overall, great food, great prices, great crowd - especially on karaoke night.
Because of its location, I recommend making reservations.
Sansei Seafood Restaurant
Waikoloa Beach Resort
Waikoloa, HI 96738