I lived in Irvine for a couple of years, I don't think I would have survived without a car. It's true that you can go to the gym and do groceries without a car, but basically that's it. Public trasportation is not really an option during weekends and at night, so if you want to go to the beach/bars/restaurants you will need one.
As for the housing options: after one year on campus, I moved to a nice apartment on the beach, in the quiet part of Newport. It is a bit cheaper than Irvine, and UCI has a free shuttle from NB to campus.