The 'De venenis' attributed to 'Malachias Hibernicus' is a portable discussion of vices and virtues. Probably composed about 1280, originally as an aid for Franciscan preachers, it adopts the innovative metaphor that sin is a poison removed by various 'treacles'. Its argumentative mode is to adduce scientific data about venomous beasts, the sins, and the antidotes to their poisons, the 'remedial' virtues. From these 'facts' of natural history, Malachy constructs homiletic similitudines (analogical figures). These, typically of a sort designed for use in sermones ad status, he applies to vicious and virtuous activities, and perhaps particularly ones peculiar to Ireland.

Although Malachy the Irishman and his On Poison have received only a handful of scholarly notices in the last century, in the later Middle Ages, his was a widely known book. A lengthy introduction presents evidence for the wide circulation of Malachy's text and the little that is known of the author. It further addresses literary issues: the work's genre, hovering between a treatise on vices and virtues, a compendium of scientific information, and a handbook for preachers; Malachy's efforts at compilation of authoritative materials; and a preliminary account of some early users, including William Langland and Robert Holcot. The introduction concludes by examining the insuperable difficulties involved in editing the text. The centre of the volume presents an annotated preliminary text and translation, together with some account of early interpolations the text received. The volume concludes with three indexes, one with all biblical citations, one of all Malachy's other citations, and a third of Malachy's similitudines, his moralised scientific information.