Irritability is a kind of signal, that an unpleasant or potentially threatening situation cannot be avoided or solved in an appropriate way.
Moderate Irritability is a common experience of tired children or overworked adults. Any kind of sleep deprivation can cause an irritable mood. Any kind of alcohol or drug addiction and withdrawal can cause irritability.
But this condition is rather unspecific and can also be caused by somatic disorders (hyperthyroidism, pain disorders, tinnitus, other chronic disorders or severe medical conditions).
Psychological causes of irritability can include mild forms of adjustment problems or stress as well as more severe psychiatric problems or disorders.
So a doctor or therapist will try to get additional information about typical situations of irritability, onset and duration of the symptoms.