<a href="http://en.wikipedia.org/wiki/Irc#Modern_IRC" rel="nofollow">http://en.wikipedia.org/wiki/Irc#Modern_IRC</a><p>If you use it through ssh/SSL it is ok.
The first 2 things that come to mind:
Whoever can connect, can log.
Even though an authenticated user's host may be masked, in the server logs they might appear in plaintext.