The current Tor hidden service address for UTPchat is wubkdspcbpabqjmb.onion
If your IRC client can handle socks5 with remote dns, you can just connect to the .onion address directly. Otherwise, use Tor's "mapaddress" feature to fake it. (We do not recommend that you use Privoxy with irssi. It's unnecessary. Just use the 'mapaddress' approach and torify irssi to start it up.) Add a line to your torrc, as in this example:
mapaddress 10.40.40.40 wubkdspcbpabqjmb.onion
Be sure to HUP (reload) Tor if you change your torrc. After you've made the change, just connect your torified IRC client to the IP you specified in your mapaddress statement for the freenode service. Tor will do the conversion for you internally and you'll connect to UTPchat (dont forget to set your proxy to tor socks proxy)
(btw .. this guide is an almost exact copy from freenode's similar guide to enter their network)
I havent tested the hidden service yet .. please tell me if it work / doesnt work .. and btw, connections from tor are slow, so bear with it if you want to use it ..