You have loaded an IRC module that monitors all IRC traffic looking for DCC Sends. It automatically changes the IP and Port in the request. Your LAN IP is replaced with the public IP, and the linux box opens its own port, and replaces the port with that port in the DCC Request. It then keeps track of where it should forward the incoming connection. When the recipient accepts, he connects to your public IP (i.e. linux box), and it connects to you, and forwards all traffic. To make it all work, switch your IP Lookup method to Normal, and everything should be transparent to your mIRC. Note that the most common implementation of this module breaks DCC Resumes, so you'll want to make sure you have the latest version, or disable it altogether and use straight port forwarding.
Sojourn
