Like Emacs and Gnus, Irssi is extendable. But unlike Emacs and Gnus, instead of Elisp, the language of Irssi extension is Perl.
Because Perl is a continuation of ancient Unix batch text processing tools - for example, sed - Perl is more difficult to read than everyday Lisp and C.
To enhance the workflow integration, and to get the familiar Elisp configuration/extention language and interface, as of ~2014, I do not use Irssi anymore. Instead, I use ERC for the little IRC I do. (I have some ERC config/extentions as well.)