As NSEU offers, a period of unaccelerated straight and level flight will bring the ATT functions of the IRS units online. 30 seconds is minimum, turbulent conditions or failure to fly straight and level can extend the time required.
This generally would mean a climb out of terminal airspace using standby AI and whiskey compass for attitude and heading reference. MAP info and FMC calculations are gone. Radio NAV systems are unaffected although presentation brings us back to basics.