My suggestion is to base if off their hostmask rather then getting their nick and whois-ing it every time. The hostmask is sent along with every thing a user says to a channel (PRIVMSG in the irc protocol), so I'm sure there's some what POE::Component::IRC returns that. I know Net::IRC does.
Comment on Re: Re: Capturing queries in POE::Component::IRC
Yes gathering the hostmask is easy enough, but I do not want to go down the route of basing it off hostmask, things like keeping hostmasks up to date and adding and removing new ones become too mundane after awhile.