Not only do you usually have your IRR obligation, but you can continue to reelist in the IRR.
There was a thread here a few weeks ago, that active duty navy serving in combat did not have to perform the reserve drills after Vietnam tours.
I have not yet seen any satisfactory documentation or explanation of Kerry's discharge-or which date is accurate. If the date is March 2001 an explanation is really needed, but no one-Rush Fox etc. has ever mentioned this.