Background: Conduct problems in children are common and have attracted considerable interest, not least because of their negative psychological, social and economic consequences. Controlled trials demonstrate that parenting programmes can be effective in reducing childhood behavioural problems, but much less is known about the processes of change or contextual factors that influence trial outcomes. Objective: This study involved a process evaluation which was nested within a randomised controlled trial (RCT) of the Incredible Years Parenting Programme (IYPP) in Ireland. The study was designed to: (1) identify and examine the key facilitative and inhibitive factors associated with the implementation of the programme in disadvantaged settings; and (2) to assess the level of implementation fidelity (IF) achieved within the RCT evaluation. Method: The process evaluation employed a longitudinal, mixed-methods approach, and consisted of two separate but related stages. In Stage One, the experiences of stakeholders (parents, practitioners and organisational managers) were assessed and explored using semi-structured interviews and focus groups. A series of in-depth interviews (N=81) was conducted with parents at pre-intervention (n=20), and at three follow-up time points, including 6- (n=33), 12- (n=20), and 18-months later (n=8). A further 16 interviews were conducted with group facilitators (n=11) and service managers (n=5) following delivery of the IYPP. Interview data were analysed using constructivist grounded theory. Stage Two was based on a mix of parent reports (N=103) and facilitator reports (N=11) designed to investigate aspects of fidelity within the RCT. Data were examined using a series of ANOVAs and correlational analyses. Results: Three overarching themes were identified from Stage One, including: (1) ‘Experiences of learned helplessness’ (e.g. the association between child conduct problems and family conflict and social isolation); (2) ‘Perceived benefits and mechanisms of change’ (e.g. the links between positive outcomes and a number of factors, including key parenting skills, social support, longer-term resilience and commitment, and facilitative organisational practices); and (3) ‘Challenges in programme implementation’ (e.g. cultural discomfort with praise and positive attention,conflict with partners; and organisational difficulties with fidelity, attrition and sustainability). The findings from Stage Two indicated that IF was high in relation to therapist adherence (M=90%, SD=4%) and parental satisfaction (M=6.69, SD=0.14), but lower with regard to the retention of parents (M=8.23 sessions, SD=4.79). There were no statistically significant relationships between IF and the primary child behaviour outcome. Conclusion: This process evaluation is one of the first studies to investigate the key short- and long-term factors associated with implementing the IYPP within disadvantaged settings. The findings underline the many benefits gained from participating in the IYPP whilst also indicating that extra supports may be required to enhance outcomes for the most vulnerable families, particularly in the longer term. Overall, the study highlights the feasibility of implementing the IYPP within the existing infrastructure of mainstream health and social service settings in Ireland. These findings represent a valuable addition to current evidence on the effectiveness of the programme, whilst also informing its routine implementation both within Ireland and elsewhere.