Mentalization-based treatment (MBT) has promising transdiagnostic applications. The evidence base for its application in non-specialist settings, including general adult community mental health services requires further evaluation. This study explores the implementation of an MBT introductory (MBTi) group in an Irish secondary mental health service.

Two online MBTi groups were delivered between 2020 and 2021. A concurrent mixed-methods design was engaged. Qualitative pre- and post-intervention measures include the Clinical Outcomes in Routine Evaluation (CORE) scale, the World Health Organization Quality-of-Life (WHOQoL-BREF) scale and the Reflective Functioning Questionnaire (RFQ). Paired sample t-test was employed to analyse change. Interviews were conducted with seven participants post-intervention and inductive thematic analysis was utilised to identify themes.

Participants exhibited hypomentalizing tendencies, which improved following the delivery of the intervention (RFQu: MD = 0.54, p = 0.032, Cohen’s d = 0.71). There were improvements across the wellbeing, problems and functioning subscales of the CORE. There was no change in the risk domain, which was low at baseline. Improvements were observed in the WHOQoL-BREF subscale of psychological health and social relationships. Five main themes emerged from post-intervention interviews: barriers and facilitators; attitudes to design and delivery; perceived intervention effectiveness; intervention coherence; COVID-19 specific issues.

MBTi delivered in a non-specialist setting is associated with improvements in mentalizing capacity. The intervention is perceived as relevant and useful by participants, although the psychoeducation and online format conferred specific limitations. The findings support the role of MBTi as a feasible transdiagnostic intervention in general adult services, as part of a range of interventions.