Background The relationship between irritability as a subjective experience and the behavioural indicators typically used to measure the construct are not known. Its links to mood, and contextual relationships, vary with age and are yet to be thoroughly examined. Aims First, to interrogate the relationship between the subjective experience of irritability and mood, and that with its behavioural indicators. Second, to determine how these relationships vary with age and over time. Method This study examined data from a previous clinical trial of adolescents and young adults (N = 82) with bipolar disorder, who received a psychological intervention over 18 months. Participants completed a battery of questionnaires, which included assessments of irritability. Analyses of covariance were conducted to examine the interaction between mood symptoms, subjective measures of irritability, behavioural measures of irritability and age over time. Results Subjective irritability scores differed significantly over time when controlling for manic, but not depressive, symptom scores. Further, subjective irritability significantly differed when controlling for behavioural measures of irritability (temper outbursts and argumentativeness). There were significant interactions between scores of depressive symptoms, temper outbursts and subjective irritability with age, wherein younger participants showed no correlation between depressive symptoms and temper outbursts. In addition, younger participants showed lower correlations between subjective irritability and both depressive and temper outburst scores, than older participants. Conclusions Subjective irritability is linked to mood morbidity and behavioural outbursts, and these relationships are contingent on age. Our novel findings suggest that subjective irritability should be assessed in greater detail in patients with mood disorders.