How does the IRS compute the true north by integrting the geographical position with the local vertical?
Something needs to provide a heading input to the IRUs during the align process. Since local vertical can be deduced in a stationary plane (vehicle) and lat/long is available (most likely from GPS), you still need to tell the units which way the airframe is pointing. Most likely from a magnetic compass and a database of magnetic north variation for geographic positions.
Once you are moving, a GPS can calculate your course, but not your exact heading due to crosswinds.