The COVID-19 emergency has affected us all, but not equally. Families where parents have mental illness (PMI) are potentially at increased risk, but little is known about how they or their support services managed under lockdown/restrictions. We harnessed our existing partnerships with adult and child mental health services in the Republic of Ireland (RoI) and Northern Ireland (NI) to investigate the qualitative experiences of service users and families in coping during the first COVID-19 lockdown (March-May 2020), and how services were supporting them. Semi-structured phone/online interviews were conducted with 22 clinicians/managers (12 from RoI; 10 from NI) who provided information from their caseloads (~155 families with PMI). Sixteen family members (10 from RoI, 6 from NI) were also interviewed. Data were analysed using standard thematic analysis. Sixty percent of families reported improved mental health, primarily due to respite from daily stresses and the "normalisation" of mental distress in the general population. Approximately 30%, typically with more severe/enduring mental illness, reported additional challenges, and mental distress including: unmanageable child behaviours; fear of relapse/hospitalisation; financial difficulties; absence of child care; and a lack of routines. Service provision varied considerably across regions. The experiences within this case study highlight unique opportunities to address the multiple stresses of pre-emergency daily living. We also highlight how mental health services and governments might become more "pandemic ready" to more effectively support vulnerable families, including addressing service overload issues, optimising the use of digital technologies, and providing in-person contact and social supports where required.