With an irregular cycle, it's likely that you aren't ovulating. If that is the case, no amount of robitussin will assist with that. The robitussin is used to help thin cervical mucus to allow the sperm to pass through the cervix and into the uterus.
I would recommend that if possible, you see an OBGYN or reproductive specialist and have them to some tests to ensure that you are ovulating, they can also give you something to help you get regular, which will assist in you planning/knowing when you will ovulate so you can plan accordingly.
If you are ovulating normally and regularly, but cervical mucus is the issue (or appears to be) the robitussin usually the first course some people choose (some doctors aren't even aware of it being used like this) because it is so inexpensive and simple.