Background: This longitudinal cohort study aimed to examine the impact of the first wave of the COVID-19 pandemic in Ireland on parents of children with externalising difficulties, in comparison to parents of children without such difficulties. Method: Parents of 159 children completed online self-report measures at three time points during the first wave of the COVID-19 pandemic; 1) Delay and Mitigation Phase (March 2020 - May 2020), 2) Reopening of Society Phase (June 2020 – July 2020), and 3) Wave 2 Case Acceleration Phase (September 2020 - October 2020). Participants were allocated to the Clinical group if they met the clinical cut off point on the Conduct or Hyperactivity/Inattention subscales of the Strengths and Difficulties Questionnaire at Time 1. Results: Parents of children with externalising difficulties experienced significantly higher levels of stress, lower levels of wellbeing, and engaged in higher levels of avoidant-focused coping strategies longitudinally. There was a significant difference between outcomes at the different phases of the COVID-19 pandemic, for stress related to parenting, personal/family stress related to the impact of the COVID-19, and type of coping strategies employed. Children with externalising difficulties, in comparison to children without externalising difficulties, showed significantly greater adjustment over time for behavioural and emotional difficulties. Conclusions: Results provide important information regarding the trajectory of psychological outcomes in parents of children with externalising difficulties over the first wave of the COVID-19 pandemic, highlighting the need for increased parental supports during, and after, the COVID-19 pandemic.