It appears to be used by, at least, ircd-hybrid and ircd-irc2. See https://codesearch.debian.net/search?q=run%2Fircd&page=3 for more references to it.

Where does it come from? Now that I’m looking, it’s actually a mystery to me. On a debian-installed system, I see an irc line in /usr/lib/sysusers.d/basic.conf which dpkg -S shows coming from the systemd package (version 247.3-6). But, on the minimal debian:bullseye docker image, there is no systemd package installed, or any sysusers-related files, but there is still an irc user in /etc/passwd. Furthermore, the source for the debian stable systemd basic.conf.in does not have an irc line.