Ireland’s practice of holding referendums to approve European treaties has operated as a significant political barrier in the European integration process. This chapter explains how the practice derives from quite distinctive interpretations of the principles of national sovereignty and popular sovereignty enshrined in the Irish Constitution. In particular, it explains both how popular sovereignty in Irish constitutional law receives a largely procedural and plebiscitary expression, and how national and popular sovereignty have become conceptually intertwined. Although both versions of sovereignty have become integral components of constitutional identity in Ireland, this chapter explains various anomalies and contradictions that arise from each.