Ireland has had an historic over-reliance on inpatient mental health care accompanied by poorly resourced community provision. There has been an increasing policy focus on provision of mental health care in the community to facilitate diversion from, or shorten stays in, inpatient care. However, little is known about the determinants of psychiatric inpatient length of stay (LOS) to allow for the targeting of community services. To use a large cross-sectional national dataset to examine the determinants of psychiatric inpatient LOS in the Irish context. Using information on 60,607 discharges from adult acute psychiatric units in Ireland between 2015 and 2019, the study employs both descriptive and regression analysis. Using negative binomial regression models, we control for patient demographic, socioeconomic, and clinical characteristics. In addition, as LOS may be related to provision of non-acute services in the patient area of residence, the analysis also controls for aspects of community service provision. The study finds that longer LOS is associated with older age, being female, and having an involuntary admission. Clinical diagnosis significantly impacts LOS, with diagnoses such as schizophrenia being associated with longer LOS. There is also significant variation in LOS across region of residence, with discharges from some regions staying up to 5days longer. The regional variation observed in inpatient LOS supports the assumption that increased provision of community services diverts or reduces the quantum of care required in inpatient settings. However, without substantive improvement in the community data collected and reported, future planning of mental health services, both community and inpatient, will be substantially curtailed.