Since its genesis, criticism of the doctrine of diminished responsibility has been extensive, both in respect of its underlying principles and practical effects. It has been called all sorts of names: “elliptical almost to the point of nonsense”,1 inaccurate2 and essentially illogical.3 Yet, in 2006, the Irish legislature deemed it appropriate to incorporate the partial defence into Irish law. To attempt to ascertain why, this paper reflects upon the development of the doctrine throughout the jurisdictions of the United Kingdom, and tracks its gradual progress to the republic under s. 6 of the Criminal Law (Insanity) Act 2006. With the doctrine now firmly enshrined in Irish law, the paper moves to consider the underlying rationale peculiar to s. 6, in addition to the early signs of its impact in practice. The first part of this paper charts chronologically the fluctuating nature and scope of the doctrine in the jurisdictions discussed, showing the malleable margins pertaining to the defence in practice. It shows how the language used to define the doctrine, and its interpretation, are shaped not so much by academic agreement on how the wording should be understood, but on political and social issues of the time. Consideration is afforded to the foundational nature of the doctrine in Scotland in order to arrive at an understanding of its original intention. The impact of its mid-twentieth-century migration to the statute books of England and Wales, and later Northern Ireland, is then discussed as this marks an important shift in the status and interpretation of the doctrine. Following this, the ascent of the doctrine into Irish law is considered, in conjunction with more recent statutory developments affecting this area of law in England and Wales, Northern Ireland and Scotland. The second part discusses in greater depth the nature and scope of the law in Ireland, in particular, its relationship with the insanity defence and its role as a means of mitigating the harsh effect of the mandatory life sentence for murder. Recent Irish caselaw is also taken into account, with a view to identifying the emergence of patterns in the interpretation of the law since its introduction in 2006. The options available to the courts at the disposal stage are shown to be lacking and the approach of the judiciary inconsistent.