Irish has a number of features such as VSO word order and initial mutations that make study of the acquisition of Irish morphosyntax particularly interesting to theories of child language development and, more recently, to language change. The chapter opens with a brief overview of Irish morphosyntax. We then outline and critically review studies of Irish morphosyntactic development over four main periods: (1) historical informal research on Irish acquisition; (2) studies of monolingual or strongly Irish-dominant acquisition; (3) a transition phase; and (4) more recent studies of acquisition in what have now become mainly simultaneous bilingual contexts. The findings of these studies are discussed in the light of the international literature and their contribution to our understanding of child language acquisition in general and Celtic languages in particular. The implications of these studies for language support and education are discussed, and future areas for research are considered.