One way of running the Mattermost IRC bridge:
docker run -d -p 6667:6667 --name ircd xyproto/matterircd -mmserver MHOSTNAME -mmteam TEAMNAME -interface 0.0.0.0
Where MHOSTNAME is the name of your Mattermost host and TEAMNAME is the name of your Mattermost team (usually lowercase).
-debug can be useful for seeing that it works.
nmap ircdhostname -p 6667 too. Adding
-mminsecure=true -restrict=MHOSTNAME might be useful for some settings.
When logging in with the IRC client, remember that the username might be an email address, for some setups:
/msg mattermost LOGIN username@domain password
Matterircd project page: github