Information is indeed scarce, which makes me wonder whether iRIS is actually a genuine brand or a rebadge from, say, an obscure oriental manufacturer (I can find no mention of iRIS in the numerous Chunghop manuals I have).

I suggest you try making enquiries here, if you haven't already, or failing that have you tried a learning remote?

Of course the long term solution would be a program of replacement, but I'm assuming that's not, presently, an option.