The example I give in the post above is for a user’s own crontab, so there’s no need to specify the username. The screen/irssi session will be started “as” that user automatically. That is, run the “crontab -e” command as “youruser”.

If you want to do something globally on the server for one or more users, then I’d try creating a file in /etc/cron.d/ and adding the “username” field, like this:

@reboot youruser /usr/bin/screen -dmUS irc /usr/bin/irssi

I hope this helps.