You have to use the good old SASL identification method.


Here's some good links:
http://blog.freenode.net/2010/01/connecting-to-freenode-using-tor-sasl/
https://itnomad.wordpress.com/2006/10/11/tor-howto-accessing-freenode-via-tor-step-by-step-for-windows/
https://wiki.archlinux.org/index.php/Tor