In January 1918, the hanging tree on the Green in Castlebar, already stooped with age, finally succumbed to the burden of the history thrust upon it when it toppled in a storm. The following year, the last of the gaols of Mayo, ceased to be a formal prison within the British prison system. The story of the several gaols of Mayo is largely untold and what is told is confused or blended with a colourful mix of half-truths. Beginning in the late sixteenth century, this study seeks to disentangle the facts from this body of folklore. The gaols at Castlebar, Ballinrobe, Prizon, Cong and elsewhere are considered in the social, economic, and political environment in which they operated including in the context of the many epidemics, famines, rebellions, and periods of agrarian violence. Over and above the incredible detail of prisoners, prison life, and the regulation and operation of the gaols of Mayo, the surviving records also contain many accounts of exceptionally cruel deeds and practices. Women, children, and the mentally ill, were subjected to the most dehumanising treatment imaginable at detention centres operated by the Mayo Grand Jury. In addition to the poor, the destitute and the bankrupt, the gaols of Mayo also held men and women who had committed some of the most heinous crimes imaginable. Between 1805 and 1919, some 196 death sentences were handed down by the judiciary at courts in Castlebar and Ballinrobe. Those sentenced to death included pregnant women, children, and the elderly. For those who avoided the gallows, dying by their own hand or terms of imprisonment, a future in Botany Bay or Van Diemen’s Land lay ahead of a long and dangerous journey.