IRC: XChat:
Go to Settings-> Preferences -> Network -> Network setup -> Proxy server and configure:
Hostname: 127.0.0.1
Port: 9050
Type: Socks5

Then make sure you check the "Use a proxy server" checkbox in the preferences dialog of the IRC server you want to use.
XChat doesn't leak DNS requests.

You might also want to check out toraliases, a small shell script you can source from your ~/.bashrc file. It defines some functions and aliases that transparently direct the traffic of some (but not all!) programs through Tor.