not irssi but I use ERC (an IRC client in Emacs) with a modified version of https://www.emacswiki.org/emacs/ErcHighlightNicknames to automatically color nicknames (it takes the first 12 characters of the md5 of each nick and treats that as an RGB value, with some clamping and color space tweaks). Everybody has a perceptibly different color, even in channels with tons of people.
It's weechat, I can't really claim all the credit, I found blog posts online about minimal setups and I've just kept it up to date. Nothing special.
Since I don't know how to easily package it I just took a tar of my .weechat and .config/alacritty directories, which should at least help you get started if you want an identical setup to mine. You just need to change your username in .weechat/irc.conf