You can get a "screenshot" of a screen session like this:
screen -p0 -X hardcopy hardcopy.txt
So this will take a hardcopy of whatever is in the first window (
-p0) and put it in a file called
hardcopy.txt (if you leave off the filename, it will use
n starts at zero an increments each time.
The screenshot is on the remote machine, and I guess you want to view it. Perhaps something like:
ssh user@remote 'screen -p0 -X hardcopy hardcopy.txt; cat hardcopy.txt'
You could also log your irssi session, so the following would work:
ssh user@remote 'tail -50 /path/to/irssi/log'
If you want a screenshot that maintains colors, then that would depend a bit on the OS and desktop environment you sshing from.