Tustin Brewing Co. Good stuff.
Head up for an evening at The Bruery's tasting room. Shouldn't be missed.
If you end up heading down towards Newport Beach, check out Newport Beach Brew Co. Also right there in Newport is the Crow Burger Kitchen -- phenomenal burgers and great tap list.
Also check out The Playground in Santa Ana. Great food, great beer.
FYI if you go to Cismontane, avoid the Cali Common. It's horrific.
Where in Irvine? I work right near the Jamboree/405 intersection... I can pretty easily get a couple homebrewers together for a quick lunch if you're up for it...