Every (active) Matrix user in a bridged room (or portal) has a corresponsing user/connection on IRC. That is, if you join a bridged Matrix room as @charcircuit:

Every (active) Matrix user in a bridged room (or portal) has a corresponsing user/connection on IRC. That is, if you join a bridged Matrix room as @charcircuit:example.com, the appservice responsible for bridging said room to IRC will open a connection with NICK/USER charcircuit to IRC (if not open already). Effectively, appservice-irc is a multi tenant IRC client.