cf - you must have missed #1934? A superb description of the system.
It would seem that once alignment is complete (and presumably kept updated by the IRUs, the Quartz Rate Sensors (modern equivalent of laser ring) will in fact provide attitude information as would an IRU. The Q is what happens if/when the IRU input is blocked or fails.
EDIT: Just seen your post - it does NOT contain any 'gyros'. Read h3's post again?