Joshua Dick and I have been putting work into improving the Irssi experience for Windows users. We made an installer built on NSIS and an USB archive made to be put on an USB stick and allow carrying your personal Irssi instance with you.
Welcome page Both packages offer running Irssi in two modes: either through cmd (the default Windows command line) or puttycyg (a Putty-based command line) which mainly looks better than cmd.
Have fun with it and report bugs please. Downloads