Modern Irish is a VSO language, in common with the other Celtic languages, and the order of elements in the structure of transitive sentences is verb-subject-object. This book provides a characterisation of the nominal, verb, clause and information structure of the Irish language from a functional perspective based on Role and Reference Grammar. We include in this analysis the layered structure of the noun phrase of Irish and the various NP operators, the layered structure of the clause and the verbal system at the syntax-semantic interface along with a number of verb valence behaviours as mediated by event and argument structure. Additionally, we survey previous treatments of Irish within a functionalist approach. The verbal noun has a special place within the Irish language and its deployment is particularly productive. We examine the derivation of the verbal noun and the contexts in which it is used. We also provide an account of light verbs and complex predicates as they occur within Irish and link this to a characterisation of the information structure of Irish. We will, in addition, provide an analysis of certain linguistically interesting phenomena that are particular to Irish (and the other Celtic languages) including the two verbs of 'to be'. Within the verbal system our concern is with the relationship between the semantic representation of a verbal predicate in the context of a clause and its syntactic expression through the argument structure of the verb. We will suggest that lexical specification is via a logical representation that reflects the aspectual decomposition of the verbal predicate and that this determines, with an actor-undergoer hierarchy, the operation of the mapping into syntax via the linking system. This book will be of interest to all linguists operating within the broad functional paradigm, along with scholars, researchers and postgraduate students interested in Irish, in particular, and the Celtic languages in general.