Background Burnout is a consequence of chronic occupational stress. Specific work-related factors may contribute to burnout experienced by those working in mental health services (MHS), many of which have increased since the COVID-19 pandemic. Aims To examine personal, work- and patient-related burnout among MHS staff in Ireland during the COVID-19 pandemic, and explore the impact of work-related conditions on burnout. Method We conducted a cross-sectional survey of three MHS across Ireland utilising a study-specific questionnaire, the Copenhagen Burnout Inventory and the Effort–Reward Imbalance scale. Results Of 396 participants, 270 (70.6%) were female. Moderate and high personal burnout was experienced by 244 (64.1%) participants; work-related burnout by 231 (58.5%) participants and patient-related burnout by 83 (21.5%) participants. Risk factors for both personal and work-related burnout were female gender, urban service, time spent outside main responsibilities, overcommitment, high score on the Effort–Reward Imbalance scale and intention to change job. Being younger, with high workload and deterioration of personal mental health during the pandemic was associated with higher personal burnout, whereas a lack of opportunity to talk about work-related stress contributed to work-related burnout. Fewer factors were associated with patient-related burnout, namely overcommitment, working in urban services and poorer physical and mental health during the COVID-19 pandemic. Conclusions High levels of personal and work-related burnout were found among mental health workers. The weak association with COVID-19-related factors suggest levels of burnout predated the pandemic. This has implications for MHS given the recognised additional work burden created by COVID-19.