You might connect your Linux box using IrLAN to another network device such as a Linux box with IrLAN, a HP NetBeamer or a Window$95 box with Infrared Network Device support.
Dag Brattli wrote: "If you want to use IrLAN you must modprobe irlan_client before ifup eth0. I had to remove the request_module() stuff since that needed a process context which I don't have in the kernel. " Maybe you have to choose the access mode. You can do this by using modprobe irlan access=1 for direct mode. IrLAN states that a provider can either be in direct mode, or in peer mode, so you currently have to choose when you start IrLAN.
Run ifconfig eth0 up <ip_address> netmask <ip_netmask> to configure it with IP-address and other parameters. If the protocol is still running you may start communicating. It is possible to use RedHat's netcfg to do this, since it makes it very easy. Next time you only need to do /sbin/ifup eth0. Notice that ifconfig does not know how to deal with IrLAP addresses, so the address is really just the 4 first bytes (in little endian format).
Test the network device by pinging to it. For detailed information about further setup see the NET3-HOWTO.
Do not forget to add a route, e.g. route add default gw <ip_gateway> or route add -host <target host> dev eth0.
Ping to another IP now, to test the connection.
For testing reasons I recommend only to use one laptop and one IR ethernet device in the same room. If there are problems look which different modes for the IR ethernet device are possible. Try them.