Since the spirit of the original post was to find out where to eat for a newcomer, I'll not limit it to Irvine (and frankly Irvine is very ordinary and too chain-oriented). I'll center it around Irvine instead.
Bentoss - A Japanese "fast food" in front of Mitsuwa just south of South Coast Plaza. The best bento joint in (dare I say) all of L.A. They're a fast food chain from Japan, but their tempura, katsu, etc. are outstanding. Also another in Gardena.
Shik Do Rak - I'll give them kudos just based on L.A. & Garden Grove locations, and because all other Koreans near Irvine are pretty mediocre. And me being Korean, gotta give at least one
Honda-Ya - A Japanese izakaya in Old Tustin that has good food and a great atmosphere. If you've never been to an izakaya-style bar, try this and you'll be hooked.
Five Crowns - Gotta admit, I love prime rib and this place does them well to very well. And a fun atmosphere to boot makes for a satisfying time.
Sunday Brunch at Monarch Beach Resort - Okay, it's not near Irvine but this meal is one of the best bargains around. $70/pp for brunch can hardly be called a bargain, but in a time when any gourmet meal will run $300+ for a couple, the $180 bill for this quality and VIEW qualifies.
Not the perfect list, but take it for what it's worth.