I had this same issue with
tmux and this was the solution. It seems it's an issue with the 256-color support or something.
So there you go, if you're using the latests
irssi and see only half of the screen refreshes (or just the last line) then this workaround will solve it for you.
I put it on my aliases list like:
alias irssi='TERM=screen irssi'