Enabling UTF-8 in Putty for irssi
From Pulsed Media Wiki
Startup your putty and go to Translations and from there select UTF-8. If you have already connected to server, you can go into settings also and change Translation the same way.
Making UTF-8 Default translation
Before you connect to server, change translation to UTF-8 and save as "Default" connection, choosing that from the list on initial page and clicking save.