IANAL but I disagree.
My understanding is: As long as you remain in the Individual Ready Reserve, you are subject both to immediate recall and to the UCMJ. Only a transfer to the Standby Reserve or a discharge ends your being subject to the UCMJ. (I will try to find a link to the exact language on this point.)
You are subject to recall in conditions of national emergency under circumstances authorized under law. You become subject to the UCMJ when you are on active duty order or under inactive duty for training orders, but you are not subject to the UCMJ for your civilian off-duty actions. I am stating this from my professional knowledge and experience as a retired Captain, USNR. I spent a lot of years on these issues.
Nope, Andy Jackson is right.
The first part is correct. IRR status makes one subject to immediate recall. Standby by reservist can be recalled as well, but it's more difficult, and may require express Congressional permission. The second part is incorrect. Reservists, whether Selected, Inactive Status, or Standby are not subject to the UCMJ. The exception being Selected reservists actually performing duty, either temporary active duty, or inactive duty training. Guardsmen are not subject to the UCMJ when performing strictly state duties. However the states have similar laws to cover them in that case.
Theres is a section of th UCMJ itself which indicates who it is applicable to.
UCM 801.2 (Note that's an Air Force site)
Just so there is no confusion, the "Fleet Marine Reserve and the Naval Fleet Reserve are the retired lists of those services. There is one significant exception. Anyone who obtains their discharge fraudulently can be tried under the UCMJ.
UCMJ ART. 2. PERSONS SUBJECT TO THIS CHAPTER
(a) The following persons are subject to this chapter:
UCMJ Art 2 (d) (1) states:
(d)(1) under section 815 (article 15) or section 830 (article 30) with respect to an offense against this chapter may be ordered to active duty involuntary for the purpose of-
(A) investigation under section 832 of this title (article 32);
(B) trial by court-martial; or