I've been using http://irssi.org for a while and it works great when I'm typing fewer than 80 or so characters on a line.
When I type more than 80 characters, however, the words I typed scroll off out of sight. In practice, this often isn't a problem if I've got a good stream of thought going. I just keep typing and hit enter at the end and all is well.
The problem comes in when I'm halfway into a long line and I realize I need to go back and edit what I've typed. I hit the left arrow and try to find the right place I want to edit. Then I right arrow back over and try to pick up the train of thought. With only about 80 characters visible at any one time, I sometimes make a mistake and introduce a grammatical error.
I'm open to a variety of solutions to this general problem. I haven't used irssi scripts before but I'm open to it.