I have recently added a patch to irssi-text to fix #274201 (Displays incorrect Chinese characters on input bar). As I don't use Big5 myself, I can not properly test the patches. I would appriciate it if anyone that does use Big5 can install http://www.davidpashley.com/debian/irssi-text/irssi-text_0.8.9-6_i386.deb and let me know if it works for them.