The basic claim of this paper is that the noticeable variation in the positioning of the Old Irish (= OIr.) perfectivising particle ro in the verbal complex can be accounted for in terms of a process of morphological externalisation. In order to make clear the extent of this claim, I first offer a brief description of the morphological structure implied by the OIr. verbal complex and I propose a classification of the OIr. forms including the particle ro. Second, I develop a notion of morphological externalisation, specifically, of its mechanism of change, according to which externalisation involves a sequence of paradigmatic reanalysis and proportional analogy, and which may serve to explain the reordering of morphemes observed both in Haspelmath’s (1993) and in Mithun’s (1999) studies. Finally, I explain the assumed change in which the particle ro changes its position within the OIr. verbal complex as an example of externalisation in complex morphological structures of the type dealt with in Mithun’s study.