The best Korean restaurants will be in Irvine, CA unless you want to drive up to Koreatown in Los Angeles. Remember that Korean restaurants have a specialty depending on what you want to eat. If you want juk, you go to a juk restaurant, If you want bulgogi, you go to a place that has BBQs.... you get the idea, I'm sure.

I also found this:

https://www.google.com/search?q=korean+restaurants+in+irvine+ca&oq=korean+restaurants+in+irvine+ca&aqs=chrome..69i57.4600j0j7&sourceid=chrome&ie=UTF-8

If you want cold noodle soup, naengmyeon, I highly recommend you go to Yuchun in LA because it is world-renowned. https://www.yelp.com/biz/yuchun-los-angeles-5?osq=Naengmyeon

Hope that helps! Feel free to email me at nancy@refreshthesoul.com if you want more off-line suggestions. I just recently moved back to the East Coast after being in So Cal for 20 years.