You are correct that converting IRS cases is the simplest way to acquire IR cases, if you can't find original ones. I believe you can save the cases you have. The problem could have been caused by improperly sizing the IRS cases, but It could have been caused by improperly seating the bullets also. If any of the cartridges can be chambered with reasonable effort, firing them will correct the problem. If any of the rest can be chambered after pulling the bullets, you can empty the powder and load it again with a fireforming load.
Firing this load should correct the problem. Those cases that will not chamber, without the bullet must be resized. A normal 8mm full length sizing die, with the expander assembly removed should work (don't forget to lube the cases). Removing the expander serves two purposes. A FL die sizes the neck smaller than required, and the expander opens it back up. Not opening the neck up for a .323" bullet should leave it small enough for .318" bullets. Also, if you forget to lubricate the cases and one sticks in the die, you can drive it out with a punch if the expander is not in the way. I suspect the cartridges you have were loaded similarly to this, but they didn't "bell" the necks to get the bullet started straight (and maybe not chamfered, inside). It is not common to bell necks for jacketed bullets, but if the necks are too small to start the bullets straight, they should be belled. I load 8x57 I/IS/IR/IRS/R-360, 8.25x57R Hagen, and 8x72R all using the same set of Lyman 8mm Mauser dies. You have to just adjust the dies to the cartridge being loaded, and sometimes be inventive (such as using a 32 S&W or ACP expander to bell necks, etc.). For a fire forming load, I load 6-10 grains of a fast pistol or shotgun powder, tamp a quarter sheet of toilet paper over the powder and fill the case with grits/corn meal/ worn out polishing media and close it off with another piece of toilet paper. Load one with a small powder charge and if the case isn't fully formed, increase the charge until the cases are fully formed.
I hope this helps, but if not, we can try again. BTY you can remove the "S" from the "IRS" headstamp.