Having used IRRToolSet in elder Ubuntu releases (elder compilers, libraries etc), i finally decided to compile it under 9.10 (yes, previously made RtConfig works under 9.10 too, «but anyway»).
But something goes wrong, with several files — illegal conversion and «bad» syntax and the like.
So, if you’ll run into the same problems, hope this patch will help.
Apply patch as usually (in the directory where IRRToolSet-4.8.5 is unpacked):
$ patch -p0 --dry-run < IRRToolSet-4.8.5-ubu-patch.diff
If everything is OK, remove
--dry-run switch and run again.