Getting irssi running with utf8 support in a putty/tmux stack is madness. Here is what you have to do.
add lines in .{ba,z}shrc:
export LANG=en_US.utf8
export LC_ALL=en_US.utf8
add lines in .tmux.conf:
set-option -g default-terminal "rxvt"
set-window-option -g utf8 on …