This study aimed to assess mental capacity for treatment decisions among psychiatry inpatients in Ireland and explore the relationship, if any, between mental capacity and various demographics and clinical variables. We assessed mental capacity for treatment decisions in 215 psychiatry inpatients in four psychiatry admission units. Almost half of the participants were female and the most common diagnoses were schizophrenia or a related disorder and affective disorders. Overall, 1.9% of participants lacked mental capacity for treatment decisions; 50.7% had partial mental capacity; and 47.4% had full mental capacity. These proportions did not differ between female and male patients. On multi-variable regression analysis, greater mental capacity was significantly associated with, in order of strength of association, voluntary admission status, Irish ethnicity, being employed and younger age. However, while these relationships were statistically significant (i.e. were unlikely to have occurred by chance), together they accounted for just 27.6% of the variance in mental capacity between participants (i.e. they were not very strong). The relatively high rate of "partial mental capacity" identified in our work suggests that decision-making supports are likely to be of substantial importance in assisting psychiatry inpatients making decisions about treatment, especially involuntary inpatients whose mental capacity is especially likely to be impaired. Future research could usefully clarify and quantify the role of cognitive and other factors in relation to the unexplained variance (72.4%) in mental capacity identified in this study; and explore which models of supported decision-making are most likely to assist the substantial proportion (50.7%) of psychiatry inpatients who have partial mental capacity for treatment decisions, as well as the minority lacking such mental capacity (1.9%).