Firstly, tell your local terminal application that you want a utf-8 window. This is left to you, but under macos (which I use), right click the window, select ‘Window settings’, pick the ‘Display’ option from the drop-down, and pick utf-8 under ‘Character set encoding’.

Next, when you start the screen session, pass the ‘-U’ flag. This has to be passed to a new screen session - you can’t connect to an existing one this way.

screen -U

Alternatively, you can turn on the utf-8 flag for a single existing screen window by typing your hotkey (ctrl-a by default), then ‘:utf8 on’. This is good if you don’t want all of your windows to be utf now.

On the remote machine, make sure that the ‘LANG‘ environment variable is set to something UTF-8 like, for instance, I use

export LANG=en_GB.UTF-8

in my .bashrc.

Finally, you need to tell irssi to use UTF-8. Start it up in your new utf-8 window, and type

/set term_type utf-8

Hopefully everything should work now.