This is a unidirectional bridge between XMPP and IRC. It is currently an instance of biboumi.
To join an IRC room, for example #channel on irc.libera.chat join the XMPP MUC #channel%irc.libera.chat@irc.cheogram.com. For more information on automatic authentication to services, and other settings, see the biboumi documentation.
Providing this service is not free, if you find it useful please consider supporting infrastructure costs on Liberapay, on Patreon, or by becoming a paying customer of JMP.chat.