It seems that I'm not as knowledgeable as I'd like to think myself. On my recent HOWTO on getting to the airport, I sent my readers on a rather winding route on Orange County's local buses to get to John Wayne Airport. In my defence, SNA is the only airport in southern California I haven't ever flown out of.
A reader in Irvine's transportation department sent me an e-mail about the iShuttle system- it's a commute-hour-only shuttle designed to take workers to and from the Metrolink station, and it provides direct service to John Wayne. If you're travelling on a weekday during commute hours, it should suit your needs perfectly. I've updated the HOWTO accordingly.