Got this from a thread on the internet:

Section 1. Configuration

Follow the following steps. Please do not use scandinavic
characters until you have proceeded configuring irssi & screen
since they probably will not work properly while the
configuration is not finished.

Step 1. Connect to the server to run your irssi on

Step 2. Shut down your irssi

/quit

Step 3. Close the screen you were running it in (if it didn’t already)

Step 4. Backup your current irssi & screen configuration:

$ tar czf irssi-backup.tar.gz .irssi .screenr*

Step 5. If you have a line starting “defencoding” in ~/.screenrc, remove it

Step 6. Start screen like you usually do, but add a -U parameter

$ screen -U

or perhaps

$ screen -U -S irc

This makes screen understand that irssi talks utf-8 to screen

Step 7. Configure irssi to talk utf-8 to screen but use iso-8859-15 charset by default for all windows

Copy-paste the following commands to the irssi console:

/set term_charset utf-8
/set recode_autodetect_utf8 ON
/set recode_fallback ISO-8859-15
/set recode_out_default_charset ISO-8859-15
/set recode_transliterate ON
/set recode ON

Step 8. Detach screen (typically by pressing ctrl-a d)

Ta-da! Now you have configured irssi and screen to work properly.