Thank you! This has worked. I also used some old settings from a past Debian installation to work out what to put in the file.
I now have Irish and Russian keyboards, switched by Alt+Shift and working perfectly. The system locale is POSIX (saving space on bootable USB stick), yet there does not seem to be a problem with Russian in Firefox. I can’t read Russian file names on the FAT32 USB stick, but it’s not a problem, I don’t really need them.
Here is my shiny new /etc.X11/xorg.conf.d/00-keyboard.conf file. I have used susestudio to add it to the appliance and it works fine.
Section "InputClass"
Identifier "system-keyboard"
MatchIsKeyboard "on"
Option "XkbLayout" "ie,ru(winkeys)"
Option "XkbOptions" "grp:alt_shift_toggle,grp_led:scroll,nbsp:none"
EndSection