To be honest with you, the Irvine Amtrak requires a guaranteed one bus transfer. The most direct would be the 79, followed by the 86. If you're willing to spend a bit more (and if it's possible to rebook your trip, it might be a good idea to instead take the 59 to the Santa Ana Depot, then take Amtrak from there to San Diego. I' aware it's more expensive, but the cost is relatively close. That way, there would be no bus transfers.