Under normal circumstances, you won't be able to get the final version of iR Shell to work with 3.90 M33. Luckily, however, there's a solution for it, which Ahman himself managed to reveal. Ahman recently provided some info for patching the PRX files so that it would remove the firmware check in iR Shell v3.9, allowing it to run on 3.90 M33.

Luckily for the less technically inclined, someone managed to do the hex editing required to get it to work. CaptainMorgan4 of DCEmu reports that all you have to do with the following files is replace the ones in your irshell/bin/ directory with the ones here, and you should be able to use iR Shell v3.9 on CFW 3.90 M33.

There are some caveats, however. CaptainMorgan4 also notes that this is unsupported and that certain features may be broken as a result, so you probably won't get the full iR Shell experience on 3.90 M33. That being said though, if you can't live without iR Shell working on your PSP, then you might as well try and see if this patch works for you.