It is not possible to know what nicknames exist in irc / slack #7
Description
Currently it is difficult/impossible to know if a nickname exists/is online in irc if you're in slack, and to know what the nickname to @ mention in slack is, from irc. This means you can't autocomplete a nickname to get the user's attention.
Equally there's no comment/guidance on a new user of IRC to know you have to put an @ sign in front of a username to get the attention of someone in slack.