The thesis investigates early Irish views on the concepts of thought and cognition through the lens of philosophy of language. It aims to establish how problems of linguistic expression relate to the understanding of mental activity in early Irish learned tradition (ca. 650–1100), particularly in such discourse-oriented disciplines as grammar and biblical exegesis. Irish contributions to this topic offer a unique perspective on the relationship between language and thought, not least due to the thriving bilingualism of Irish intellectual tradition. Therefore, this study brings together Latin and vernacular evidence and traces links between ideas expressed in both languages. The study has a tripartite structure which moves from the views on the material aspects of language, towards Irish theories of meaning, and onwards to ideas that imagine thought itself as a special kind of language. The first part centres around Irish approaches to phonology, writing systems and criteria that define a word. It aims to explore the ways in which Irish grammarians considered the material aspects of language to establish basic mental mechanisms for the creation and processing of meaning. Part two surveys evidence for Irish theories of signification and investigates problems of the relationship between form, meaning and thought. The final part considers Irish language-philosophical theories which connect language and cognition, namely the techniques of non-literal exegesis and the concept of ‘mental speech’ – a metaphorical device which presents thought patterns in terms of language patterns. Overall, the thesis offers the first comprehensive study of the intersection of philosophy of language and philosophy of mind in early Irish intellectual tradition.