You can easily ask the staff from feralhosting to install the plugin, just open a ticket and require this plugin.

I opened ticket and this answer

I am sorry but we are unable to provide support for third-party software.

Also, if you want to do it yourself go here: autodl-irssi | Free Communications software downloads at SourceForge.net and read more about it. With the details that feralhosting gave you access ssh through putty and install autodl plugin according with this thread steps: [HOW TO] Setup Autodl-irssi on a SeedBox

Check this link: http://www.youtube.com/watch?v=ma6Ln30iP08 to view how to connect to SSH tunnesl with putty.

Download and install putty from here: http://www.putty.org/

I do not know how to use putty
Don't know the use of Linux systems