This paper sets out to identify the categories underlying Irish verbal inflection and to explain why they have their observed morphological and semantic properties. Assuming that the semantic range of a tense is a function of the whole clause, it derives the tenses of Irish from three syntactic features. Their basic value and position in the clause, along with that of other independently justified formatives, determines the attested range of interpretations for each tense, while the way they are spelled out determines the observed morphological patterns. Since the analysis of verbal categories is based on their syntactic realization, the same explanation accounts for the paradigmatic structure of Irish conjugation and for various syntagmatic phenomena of contextual allomorphy. A language-specific investigation thus claims a broader theoretical significance as an exploration of the interconnected workings of syntax, morphology, and semantics.