As Lpfix5 said, there's no way to track someone by IP through the IRC Server without being on a channel with that person. If you have the persons "IP" and you think the person might be in the same channel as you, but on an unknown nick (on DALnet), you could possibly use this:
/who +Mh some.host.or.ip.here.com
This can also use wildcards. ie: /who +Mh *.verizon.*