Ok, yet another try, this one will do the trick I hope... ;)

Credits are due to reverendj1, who described a way of assigning a Custom key to toggle keyboard layout, using setxkbmap, which featured in the workaround alluded to in Edit 2 of my previous answer...

Here is the slightly more involved way, but will give you more
flexibility for the key combination. First, copy and paste this script
I wrote and name it something like kb_toggle.sh. Make sure to edit
LANG1 and LANG2 to be the keyboard layout codes you need.

#!/bin/bash



LANG1="us"

LANG2="de"

CURRENT_LANG=$(setxkbmap -query | tail -n 1 | cut -f6 -d ' ')

if [ "$CURRENT_LANG" = $LANG1 ]; then

    setxkbmap $LANG2

else

    setxkbmap $LANG1

fi

make the file by right-clicking on it -> "Properties" -> "Permissions"
tab, then select "Allow executing file as program.

Now, open "System Settings" -> "Keyboard" -> "Shortcuts" tab and
select "Custom Shortcuts". Click the + button on the bottom and name
the shortcut "Keyboard Toggle" or whatever you want, really. Then give
the full path to the script you made earlier in the command box. Hit
Apply. Click where it says "Disabled" then you can set the shortcut to
whatever you want by clicking your key combination!

I think you'd need to change "de" to "ir".

Again, good luck!

Edit

You may even get away with a one-liner, if you prefer:

setxkbmap us,ir -option "grp:alt_shift_toggle"

Now you can switch by pressing alt+shift

See also http://wiki.lxde.org/en/Change_keyboard_layouts and http://www.x.org/releases/X11R7.7/doc/man/man1/setxkbmap.1.xhtml

Edit 2

Depending on how you decide to achieve your goal, you may want to include lv3:ralt_alt in your command, resulting in

setxkbmap us,ir -option "lv3:ralt_alt,grp:alt_shift_toggle"

See man xkeyboard-config | grep lv3 for details:

lv3:ralt_alt

Right Alt key never chooses 3rd level

   xkeyboard-config  provides  the  description files for the X Keyboard Extension (XKB). The configuration options below are

   usually applied with setxkbmap(1).