The three extant versions of the pseudo-historical prologue to the Senchas Már share an original core best preserved in the Harley recension. Its centre-piece, an archaising poem ascribed to Dubthach maccu Lugair, stands revealed in translation as a sophisticated scripturally-based argument for punishment of culpable homicide by death in spite of the christian doctrine of forgiveness. As such, it is integrally bound up with the surrounding prose ascribing the foundation of early Irish law to the fusion of native legal with imported biblical concepts under clerical auspices symbolized by St Patrick. Despite its bogus appearance as commentary, the prose must be contemporary with the poem, which is unlikely to be post-eighth-century on linguistic and stylistic grounds but is hardly much older either on the evidence that Muirchú’s Life of St Patrick was its main source. This earlier dating of the prologue goes hand in hand with further evidence for the recent revolutionary contention that so-called rosc composition is not necessarily an archaic, oral and pagan phenomenon but could be produced by clerics working from written Latin sources as late as the eighth century. An annotated text of Dubthach’s rosc concludes the discussion.