The Irish Mental Health Act (2001) is undergoing revision. In 2014 an Expert Review Group recommended that the term currently used in the act “mental disorder”, should be replaced with the term “mental illness”. We argue that the proposed change, while well intentioned, contradicts the internationally adopted terminology of “mental disorder” used by the United Nations, World Health Organisation and European Commission. The term “mental illness” is atavistic, it implies an unsupported cause, it contravenes the United Nations Convention on the Rights of Persons with Disabilities (UNCRPD), and it is associated with concerns regarding stigma and reduced self-efficacy. Furthermore, the term “mental illness” is not used in any internationally accepted diagnostic or classification system in the mental health field. While any term used to describe mental health problems, may be contested, Ireland should not revert to using archaic terminology. In accordance with international best practice, and perhaps in lieu of a willingness to accept more progressive alternatives, Ireland should continue to use cause-neutral terminology, such as “mental disorder”, in the revised Mental Health Act.