The thesis examines the extent to which medicalism and legalism have influenced the legal powers of detaining the mentally ill in Ireland from the late eighteenth century to date. Utilising academic literature, government publications and original interviews with treating psychiatrists and service users, the thesis provides the first comprehensive socio-legal analysis of the law concerning mental health detention, the interpretation of the law and the operation of the law in Irish society. It transpires that a specific approach to the legal powers of detention became embedded in Irish society‘s response to mental illness. This approach provided psychiatry with significant power in the detention, care and treatment of the mentally ill and society ardently relied on psychiatry to manage the perceived social problem caused by mental illness. Simultaneously, legal powers of detention dominated by medicalism have been ingrained in the Irish psyche. Over time, however, legalism evolved as a result of the widespread critique of psychiatry‘s role in the detention of the mentally ill. Furthermore, a strong line of international law established that the rights of the mentally ill in detention required greater protection. These developments were subsequently enshrined in the Irish law governing the legal powers of detention and new provisions were introduced that reduced psychiatry‘s control in the detention of the mentally ill. Despite these changes to the law governing the legal powers of detention, it has been found that further movement in the direction of legalism will require a more comprehensive shift among those applying the law, in particular, the judiciary and the psychiatric profession.