Module - shabble/irssi-docs GitHub Wiki
Irssi loadable modules are quite different from scripts, in that they require compilation and are much less likely to be available "as-is" for your particular setup. They are typically supplied as source code, which must be compiled before loading.
They are also more powerful than scripts, since they can access any of Irssi's internal functions and datastructures, rather than be limited to the exposed scripting API.
They are generally used to provide scripting support in other languages, such as:
First, make sure that whatever you're doing can't be done with a script. Scripts are far more portable, easier to write, debug, and distribute.
A basic skeleton of an Irssi module can be found at https://github.com/shabble/irssi-scripts/tree/master/modules/test 
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Modifying it to do your evil bidding is now up to you.