Pirch has the built in ability to use the SOCKS5 support that is built into SpoonProxy. (Keep in mind that SOCKS5 support in SpoonProxy expires after 30 days in the non-registered version. To re-activate it after that period, you will need to register.)

In Pirch from the File Menu there is an option for Proxies. Select it and you'll get the following dialogue box:

As shown in the screen capture above, check the box that says Enabled, specifying that you wish for Pirch to use a SOCKS Firewall to connect to the internet.

In the SOCKS Host, put in INTERNAL IP of the machine you're running SpoonProxy on. The default port is 1080 and should remain so unless you have changed it for some reason.

You should then be able to connect to IRC normally through the Proxy.