There is a USB IrDA Bridge Device spec at Rev 0.9B , it's being adopted as a USB class specification. You can find it at under 0.9 Class Specification header.
As far as I know the Actisys 2000U and Extended System ESI-9685 dongles seem to be based on the same hardware. Both USB dongles work fine with the Linux driver. It's possible to have multiple USB dongles in a box (for now, only up to 4).
The driver is not yet in the kernel and not yet super stable. All current hardware are the same and not that great, IMHO. The driver is available in the latest IrDA patch. Startup with :
modprobe usb-uhci modprobe irda-usb irattach irda0 -s 1