On plenty of networks (it's been years since I was on IRC, so unsure which IRCd's support it, but iirc both Quakenet and Freenode do in slightly different ways) even support host-masking as long as you are auth'd on the network - of course, IRCops could still find out, so a subpoena (or hacking into the servers) could see it, but prevents /whois from telling you at least.

(I think some networks even partially hide your IP by default anyway)