In order to examine institutional change in Republic of Ireland (hereinafter referred to as 'Ireland' or 'the Irish case') as a consequence of European integration, this article looks at the adaptation of national institutions subsequent to EU membership. A new institutionalist approach is taken towards the definition and discussion of institutions in order that the analysis may encompass the broadest range of changes consequent to EU membership: in relation to national structures; decision-making patterns; socialization processes; and overall 'system change'. The analysis uses the organizing concepts offered by Bulmer and Burch (1998) in their evaluation of the impact of Europeanization on national systems of public administration in Britain. According to this framework, 'institutions can be analysed in terms of four gradations moving from the formal, through the informal, to the normative and cultural' (Bulmer and Burch 1998, p. 604).