There is a well-populated operating systems development IRC channel on Freenode at #osdev. Several regular contributors to the OSDev.org wiki and forums are usually present in the channel.
IRC client suggestions
Make your own! IRC is one of the simpler protocols to implement, to the point where it could be the first thing you implement after TCP and Telnet. If there's anything tricky about it, it's calculating the maximum length of the actual text, but you can just use a fixed maximum which allows room for all the other fields at their maximum lengths. Or just ignore it, Freenode seems to handle long messages well. In any case, the standard is RFC 1459
Freenode offers an AJAX IRC web client in case you don't have or want to make one.
- ii is a file- and FIFO-based IRC client for POSIX systems in under 1000 lines of source code
- IRSSI is a popular and powerful text-mode IRC client.
- Pidgin supports a vast range of different chat services in one client
- Riot has Matrix and IRC in one package
- sic is under 250 lines of C for POSIX systems
- WeeChat is a somewhat popular rival to IRSSI, but contrary to its name, it has been known to use many times the memory and CPU of IRSSI
- wircrc is a multi-window IRC client in under 300 lines of Plan 9 shell script, which shows what can be done with operating system support
- xchat a X11 based IRC chat client, somewhat user friendly and available in many Linux distributions.
See also Wikipedia's Comparison of Internet Relay Chat clients.
Somewhat related: Bitlbee is an open-source gateway from IRC to many other chat services. It could be a shortcut to using these other services with your OS. It uses the same libpurple as Pidgin, but obviously cannot support images or video.
There is an active and welcoming OS development server on Discord with hundreds of members. The server is not affiliated with this OSDev.org.
Invite link: https://discord.gg/RnCtsqD
They also have a matrix channel: #osdev:matrix.org