Background: Parental mental illness is common and can lead to dependent children incurring a high risk of developing mental disorders, physical illness, and impaired educational and occupational outcomes. Family Talk is one of the better known interventions designed to prevent the intergenerational transmission of mental illness. However, its evidence base is small, with few robust independent randomised controlled trials, and no associated process or cost evaluations. The PRIMERA (Promoting Research and Innovation in Mental hEalth seRvices for fAmilies and children) research programme involves a mixed methods evaluation of Family Talk which is being delivered in mental health settings in Ireland to improve child and family psychosocial functioning in families with parental mental illness.Methods: The study comprises a multi-centre, randomised controlled trial (RCT), with nested economic and process evaluations, to assess the clinical and cost effectiveness and implementation mechanisms of Family Talk compared to usual services. The study is being conducted in 15 adult and child mental health settings in Ireland. Families with a parent with mental illness, and children aged 5-18 years (n = 144 families) will be randomised to either the 7-session Family Talk programme (n = 96) or to standard care (n = 48) using a 2:1 allocation ratio. The primary outcomes are child psychosocial functioning and family functioning. Secondary outcomes are: understanding and experience of parental mental illness, parental mental health, child and parental resilience, partner wellbeing and service utilisation. Blind assessments will take place at pre-intervention, and at 6-, 12-month follow up.Discussion: Given the prevalence and burden of intergenerational mental illness, it is imperative that prevention through evidence-based interventions becomes a public health priority. The current study will provide an important contribution to the international evidence base for Family Talk whilst also helping to identify key implementation lessons in the scaling up of Family Talk, and other similar interventions, within routine mental health settings.Trial registration: ISRCTN Registry, ISRCTN13365858. Registered 5th February 2019. http://www.isrctn.com/ISRCTN13365858