Light rail is not going to be economically sustainable without denser development. I don't really see denser development coming to Irvine.
Don't get me wrong--as an occasional Metrolink rider and someone who likes taking Metro in LA, I would love to have light rail. But I am realistic about why it wouldn't be economical given TIC/Pereira's master plan.
Maybe in 20 years, gas will quadruple in price, the Jamboree corridor will get denser, and we'll see a route from Marketplace-Tustin Metrolink-Diamond Jamboree-Park Place-JWA-Newport Center (Fashion Island). This line will either be super slow, because it is at grade, or people will sue the hell out of it because its elevated guideway will block someone's view.
The only realistic plan I've seen for rail in OC is the Santa Ana Streetcar from their train station to DTSA, and even then, that thing is going to be slow as all hell. Boring.
edit: and Anaheim between ARTIC and Disneyland, but that's really for tourists