I have a feeling this isn't something that is being developed anymore because of technologies like BlueTooth. Maybe some fiddling in the terminal could fix this(?). I remember reading that the IrDA port on PowerBooks was for things like IrDA modems or cell phones with infrared ports, not for file exchanges.
This seems to match up with what I've seen. I did a little searching around on the web, and it seems like only Apple in Europe ever advertized any kind of IrDA adapter for the Mac. There is a driver in one of my library folders, and I get the impression from what I've read on the web that it is for some USB IrDA adapter sold by Belkin (SmartBeam), but only in Europe. The sites I have seen talking about IrDA on the Mac say it uses a chipset from KC Technology and some other adapters with that chipset might work.
In the USA, seems like MadsonLine is the only company that is/was selling an IrDA adapter for the Mac
I can't seem to find a link to the adapter from their main page, so maybe they've dropped it.
If you say file transfer doesn't work, I'll take your word on it. It wouldn't surprise me. Apple does not seem to want to admit that they support IrDA at all anymore. Since your Powerbook actually has an IR port, do you have any support contact at Apple you could ask about it?
I'm sure someone who was motivated could write some better drivers for the Mac, or port the Linux drivers over, but the best we could get from that is the ability to beam vCard/vCal items to/from the Zaurus. It would be nice to have, but a real sync solution would probably be a better investment of time.