It looks like many of the irb issue(s) have been addressed now in 15.1X49-D50, or at least have PR's logged against them in the release notes. I have a working irb now post-upgrade on an SRX320.
It's probably worth noting as well the default mode for the 3xx devices is transparent-bridge, so to use your irb interface, you'll need to set this to global-mode switching, and reboot the unit.