When can a girl with irregular periods have pregnency test done?

Anytime. The hormone that triggers a positive test is not dependent on the pattern of menses, although most suggest to test after missing an expected period. The hormone is made by the placental tissue that establishes the pregnancy. It may be a low levels that won't result in a positive test early in the pregnancy, but in time it will.

Related Questions

I have irregular periods but never normally this long, I'm 8 and a half weeks late and have done 2 pregnancy tests which have both been negative?

Needs evaluated. Urine pregnancy tests while quite accurate depend on the particular hormone being in the urine. This in turns depends on the level of hormone in the blood [varies person-person and length of gestation] and how the kidneys are functioning. Many pregnant women have neg urine for as long as two months. A blood pregnancy test is completely reliable. Perhaps pregnancy needs ruled out. Read more...