Hi,
It looks like I won't need to bring my 1944 brass binnacle with Sperry Gyrocompass (to find True North) on the flight deck any more.
This Honeywell link (see page 1.9)
"Rapid Dispatch Option
If extremely rapid dispatch is required, the operator may also elect to use the Align-in- Motion function to complete the alignment in flight. When the IRS is powered-up, the attitudes, accelerations, and rates are available within 5 seconds. If ARINC label 043 (Set Mag Heading) is received from the FMS once at power-up, then all TSO outputs will be available for dispatch. When the IRS completes the align-in-motion, all parameters will be available at Full Performance as specified in section 6.0."