Comment 3 for bug 60098

After further testing it appears that Xchat chops long lines into multiple smaller lines and then pastes them. This seems to be a feature of Xchat to account for a limitation on the length of lines in irc (at least on irc.freenode.net) to 512 characters. Gaim, irssi, and ircatwork.com all appear to not take this limitation into account.

I have found a plugin for irssi that (http://irssi.org/scripts/scripts/splitlong.pl) that should take care of this.

Download the file and copy it to ~/.irssi/scripts/autorun and then in irssi run "/script load splitlong.pl"