This study sets out to analyse the pervasive and profound impact of European integration on national patterns of policy making within the core executive. As a comparative study of the UK and Ireland it explores how national EU policy is coordinated and projected, and explains how and why this process has changed over the past decade. The study aims to shape the nature and direction of research in this dynamic field in two key respects. First, it makes an innovative contribution to the concept and analysis of Europeanisation within the core executive, delineating between five distinctive modes through which change may be induced. It does so by rejecting conventional 'top down' accounts of domestic change in favour of a 'bottom up' research design that allows us to attribute relative causation to European integration. Second, the study adds considerable value to existing empirical accounts of domestic change by employing a distinctive strategic-relational network framework in order to map the changing face of policy making within the UK and Irish EU policy networks, and to evaluate and explain the impact of adaptation over time and its potential implications for policy outcomes. This provides a far more dynamic picture of domestic adaptation.