We have had people successfully bridge IRC/Zulip, though I don’t remember if it was something they hacked together or something on our side (I’ll check and get back). I’m not aware of anyone bridging Discord and Zulip.
The IRC bridge did not mirror users. So if I (rishig) sent a message in IRC, it would show up in Zulip as sent from the IRC bot, and the message body would just start “rishig: …”
The main issue with bridging though (and the reason we haven’t pursued it seriously) is that the Zulip conversation model and the IRC conversation model are kind of different. E.g. if someone replies to a message in IRC, it’s not clear how we would rethread that message in Zulip.
It’s worth noting that we do have proper bridging with both email and Zephyr; both are tools with a similar conversation model to Zulip. My guess is it would be possible to bridge with internals as well (at least on the Zulip side).