ircII help: set insert_mode

Usage: SET INSERT_MODE [ON|OFF|TOGGLE]
  Turns ON or OFF insert mode.  While on, characters typed are
  inserted into already existing text.  When off, new characters
  overwrite existing ones.  Whichever key is bound to the function
  TOGGLE_INSERT_MODE will toggle this setting on and off. The TAB 
  key (or ^I) is the default binding.

See Also:
  BIND TOGGLE_INSERT_MODE