Bottom line: In the IRR you're still in the Marine Corps Reserve and still subject to recall. The Corps mainly needs to be able to find you and keep your records current — so most obligations are about staying reachable and responsive.
Your core obligations
- Maintain current contact information — the single most important one; recall and correspondence depend on it.
- Respond to official correspondence from the Marine Corps.
- Complete required administrative actions when directed.
- Attend an IRR Administrative Muster if ordered.
On musters and recall
IRR Marines may be subject to recall under federal law, and musters are how the Corps keeps your readiness and records current. If you're ordered to a muster, attendance is required unless the Marine Corps Individual Reserve Support Activity (MCIRSA) excuses or reschedules you. See Do I have to attend my IRR muster? for the specifics on that.
Full details and contacts are on the official MCIRSA / IRR page.