IRC is a protocol and specification. There are many projects that independently implement it and thus there isn’t any specific IRC source-code.

irc itself doesn’t have source code, it’s just a set of instructions on how servers and clients should communicate. There’s stuff like this: ircv3 which defines how ircv3 works. and gives info on how the old school irc stuff worked as well.

IRC is a protocol, there’s implementations of it like Librechat: https://libera.chat/contributing/development/

The IRC protocol is defined by a couple of RFCs, the first one being RFC 1459.

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Maybe you mean the various IRC apps’ source code? That is per app. IRC itself is just an open documented protocol (so not source code at all). More info on IRC’s protocol standards at https://ircv3.net/irc/.

You have a selection of them here: https://ftp.funet.fi/pub/unix/irc/