Mattermost is an open source project that benefits from users building integrations to address the needs of their teams, and sharing back those integrations with the community.
The community has created multiple integrations to connect Mattermost to IRC, including Matterbridge for connecting a single Mattermost channel to IRC using outgoing webhooks, and Matterircd, which offers access to Mattermost through a minimalist IRC server using the Mattermost Web Service API.
There are existing requests from the community for connectivity to XMPP and Matrix that can be upvoted, and the filing of additional requests for upvote is highly encouraged so the potential user base of new integrations is visible to potential contributors.