Researchers have been investigating the clinical judgement and decision-making of nurses for several decades now. However, prior to the research described in this thesis, Registered Psychiatric Nurses (RPNs) working in the Irish Republic had not been the subject of a comprehensive study looking specifically at their clinical judgement and decision-making. As this is the first study conducted in this area, it takes an exploratory descriptive approach. With a comprehensive review of the literature and pilot study (n=7) as its basis, a novel mixed methods study was designed. Simulated cases presented in an audio-visual format were used to collect in-vivo and retrospective data in the form of narratives from participants (n=40) across the Irish Republic. The sample comprises RPNs across all levels of experience working in several sites, representing the full range of Irish mental health services. The data were analysed using comparative keyword analysis and conversation analysis informed discursive analysis. Quantitative and qualitative analysis reveals participants’ judgement and decision making to be routinised and habitual, hinging on reference to typicality grounded mainly in psychiatric diagnoses. The role of participants can be seen to represent the paternalism of the social order of which it is part. Participants express confidence and certainty in their judgement and decision-making, even where paraverbal and other discursive evidence points towards situations characterised by uncertainty. The study’s findings are of particular interest given the direction envisaged for the profession of psychiatric nursing by leading academics, health service providers and professional and statutory bodies. The findings of this study suggest that if psychiatric nursing in the Irish Republic is to proceed towards more person-centred, autonomous practice with a stronger therapeutic focus, dramatic restructuring of psychiatric nurses’ roles will be required. In conclusion, the thesis discusses this situation with reference to the challenges made evident by the study, along with the viable options available to address them.