If you're not getting periods, it's possible that there's something else wrong. Have you been to a doctor to get looked at? If you're not having a period, it's likely you're not ovulating as well. Make an appointment to get seen, and they will be able to start you on medications to help jump start your body.
People have irregular periods for any number of reasons, from the simple (such as low body fat) to the complicated. As Clysta says, check with your ob-gyn and see if any tests can be run to determine why they are so irregular.
That said, you could get pregnant before a period comes. A person has a period 14 days after she ovulates. The fact that you do occasionally have periods suggests that before each one (i.e., 14 days before), you ovulated. So if you and your husband happened to have had sex on one of those days, you'd have the normal chance of getting pregnant that cycle, i.e., the same chance any fertile couple does when they have unprotected sex during ovulation, which is said to be about 20%-25%.
I realize that you guys would probably get pretty irritated having to be sure you have sex every two or three days from now until doomsday just on the chance you happen to be ovulating, but you could use the temperature-taking method and see if you can track any changes.
However, after ttc for 5 years with no success, I think I'd cut to the chase and get some testing and advice from a specialist.
yes, It depends though...but I know that women can still get pregnant even if they don't have a regular period.