The aim of this study was to investigate the validity and reliability of the Irish Nursing Minimum Data Set for mental health to determine its usability in the clinical setting. A secondary aim of this study was to explore the ability of the tool to capture nursing sensitive outcomes of care, conceptualised and defined according to change in the patient’s condition mediated by nursing interventions. The research methodology was guided by a measurement error concept map. The validity of the Irish Nursing Minimum Data Set for mental health was established through the implementation of a number of studies to test for construct validity, content validity, face validity and discriminative validity. The reliability of the Irish Nursing Minimum Data Set for mental health was established through tests of internal consistency, factorial stability and interrater reliability. A secondary analysis of the study data was carried out to establish whether the tool could be used to investigate nursing sensitive outcomes of care. This analysis was guided by a model of nursing role effectiveness and implemented using structural equation path analysis. The overall findings of the study inferred that the Irish Nursing Minimum Data Set for mental health possessed relatively good levels of construct validity, content validity, face validity and discriminative validity. Further research is required to add to the knowledge base regarding the construct validity of the tool in particular. While some level of reliability of the tool was established, further investigation of its interrater reliability is recommended. The findings of the outcomes analysis inferred that the Irish Nursing Minimum Data Set for mental health has potential to yield useful information regarding the unique contribution that mental health nurses make to patient/client outcome achievement.