With the information received from /userhost, what do you want to do with it? There are many things you can do with it, which probably is any mIRC command.
Also, just to make note, if you're on UnderNet or any other network with masked hosts, and they have a *!*@*.users.undernet.org, it will show as the users.undernet.org host, NOT their real host. Some people have probably gotten confused by this.