Most networks these days mask IP addresses and/or allow for virtual host addresses, thus, unless you have IRCops on the network, you probably can't see the IP address.

Some networks support using an address in the /notify command.

The reason I gave earlier is the most common reason why a script for checking IP address fails more often than it succeeds.

Here's a quick script, but I don't guarantee that it will work.
Code:
on *:join:#:{
  var %IP.Address = Address1 Address2 Address3 Address4
  if $istok(%IP.Address,$address,32) {
    .echo -a $nick using address $address has joined $chan
  }
}

Replace Address1 Address2 Address3 Address4 with the Address(es) that you want to monitor. You don't have to have multiple addresses, but if you do, make sure that there is a space between each address.