Prepare Linux/IrDA as described above. Especially check for the existence of /dev/irlpt* (if it doesn't exists do as root mknod /dev/irlpt0 c 161 16). Now you may perform a first and simple test. Try to write a small file to /dev/irlpt0 by cat FILE >/dev/irlpt0. Do not wonder about a bad format (the lines form sort of steps) this is just a first check. If this doesn't work please check the permissions of /dev/irlpt0. Watch whether the connection indicator of your printer shows activity, e.g. the green light above the InfraRed port of a HP 6P/MP comes on (lower left hand corner, near the paper tray).
The cat command will not produce formatted output, but is useful for testing. If it works, you may set up an IrDA capable printer depending on your printer system. See the documentation, e.g. the Printing-HOWTO from LinuxPrinting.org for detailed information.
With the Common Unix Printing System - CUPS use for example with a HP LaserJet 2100:
lpadmin -p IRDA_PRINTER_NAME -v parallel:/dev/irlpt0 -E -m de/hp2100_6.ppd.gz
Of course other printing systems will also work, e.g. you may edit /etc/printcap and include irlpt0 as the printer device.
The better way is to change your /etc/printcap to use /dev/irlpt0 in addition or instead of /dev/lp1.
For easy printer setup you may use a printing software like APSFILTER, MagicFilter EZ-Magic (with RedHat there should also be a GUI for this purpose). Make a copy of /etc/printcap before.
Example for APSFILTER with a HP 6P (non-postscript, HP 6MP is with postscript). The two relevant questions are: "Do you have a (s)serial or a (p)arallel printer interface?" Answer "p" "What's the device name for your parallel printer interface?" Answer "/dev/irlpt0"
Restart the print daemon with kill -HUP <PID of lpd>. If you use another print daemon choose the according command.