The IP address in the listen line specifies an address that Unreal will bind to for listening. Generally, * should work, which will let the OS choose an appropriate address, but if you are on a shared server, you should use the IP address assigned to it to avoid conflicts with other IRCds running on the same server.
If Unreal is not accepting your IP address, this is a problem with your allow {} blocks; listen has nothing to do with it. Also check your ban blocks and any TKL bans.
Just a minor correction for aquanight's reply, *:6667 makes Unreal to bind a socket to all network interfaces on port 6667. This includes 127.0.0.1 (the loopback device, localhost) and every other IP addresses the OS have.