Bottom line: If you're ordered to a muster, attendance is required — but it can be excused or rescheduled. Contact MCIRSA before the date if you can't make it; don't just no-show.
When it's required
An IRR Administrative Muster is a once-a-year administrative event the Corps uses to verify your records and readiness. IRR Marines remain members of the Marine Corps Reserve and are subject to recall under federal law, so when you're ordered to a muster, attendance is required.
How to be excused or rescheduled
Attendance is required unless the Marine Corps Individual Reserve Support Activity (MCIRSA) excuses or reschedules you. If you have a conflict — work, travel, a genuine hardship — the move is to contact MCIRSA ahead of time to request an excusal or a reschedule, not to skip it.
The bigger picture
The muster is one piece of your broader IRR obligations — keeping your contact info current, responding to correspondence, and completing required admin actions. Details and contacts are on the official MCIRSA / IRR page.