open your ~/.irssi/config file in your favourite text editor, and search for the key statusbar -> items -> more. It should read more = "-- more --";
modify it to read
more = "%_%R%F-- MORE --";
you can also change the displayed text. For more colours/attributes, check the Formats page
save the file, and in irssi run /reload
to load it. (Not all terminal emulators support flashing)