Old Irish has three categories at its disposal for the formation of secondary verbs: the ā-verbs, the ī-verbs and the -igidir verbs. In this article, I discuss the possible origins of these formations before moving on to a discussion of the underlying motivation for ā-verbs and ī-verbs to be formed in one verb class rather than another. Secondary verbs contain denominatives, deadjectivals and deverbal verbs. There are no deadjectival ī-verbs and no deverbal ā-verbs or igidir-verbs. The formation of a denominative as an –ā- or an ī-verb appears to be motivated by its semantic causativity and iterativity and its transitivity. The -igidir category, on the other hand, is so productive that it appears to have virtually no restrictions in Old Irish and has been left aside in the discussion
