Kennedy Meadows is great, but its a long drive from Irvine.
I grew up in Orange County but lived in the Bay Area most my life and had the pleasure of riding the good spots in No Cal. In fairness to SoCal riding areas, I don't do much riding since we moved to Bakersfield and started having kids. But if you are looking for an area as good as Stonyford within a short drive of Irvine, you are going to come up way short.
In the National Forest near the Hungry Valley SVRA there are some unmarked (illegal) billy goat type trails. Forget the SVRA - it has a few short stretches of single track trail but is vastly inferior to what you are used to. The marked trails in the Los Padres forest next to the SVRA are better, and some are similar to trails you would find at Stony or Middle Creek, except they are beat up, whooped out and dusty most of the year. BTW, they close the forest trails whenever it snows or is wet (BOO!). Oh yeah - red stickers enforced, too.
Sometimes I ride at Ballinger Canyon - its where Kern, Santa Barbara and San Luis Obispo counties meet (long drive from Irvine). Probably 75 miles of marked trails - maybe 25 miles of decent single track. Quite fun if you catch good condtions, but not even in the same ballpark as Stonyford (can you tell I miss NoCal?).
Some of the desert riding can be quite technical and probably your most reasonable option for day trips.
http://www.employees.org/~calridng/allsites.html is a link with a list and map of most of CA's riding areas.