Nature vs. nurture in irritability?
You will have to investigate quite a few factors:


There must be a genetic/epigenetic factors to this because we know that personality trait neuroticism has genetic and environmental factors.
You should also investigate into stress - biological and psychological stress.
Investigating theories of psychological development can also come in handy, especially, Freudian theories.


In my personal view, I think a combination of genetic and environmental factors should play role in shaping irritability tolerance, however, additionally environment plays a role of stressor.

In other words, you should have genetics, epigenetics and upbringing as a predictors of general irritability. Immediate state of being can also have influence, but stressor is like a trigger.