How much time do you have?
It's possible to do this by public transportation but it will take a while. SNA is the least transit accessible airport in LA area.
Do you really want to attempt this?
http://maps.google.com/maps?saddr=sn...h&z=11&start=0
I take Metro in LA all the time and even I wouldn't try this. Public transportation option in Orange County blows.
One-way rental car is the way to go if Irvine Fly Away schedule doesn't work for you.