[Arathorn]

I'm not sure anyone has written an irssi plugin specifically, but there's an excellent weechat plugin (https://github.com/poljar/weechat-matrix). As others have said, there's also matrix-ircd which exposes all of Matrix as an IRC server you can connect to from whatever IRC client floats your boat, but you end up speaking the subset of Matrix which can be represented as IRC, which is a bit of a shame.