Ireland and the United Kingdom: A Century of Relations (1921–2021)

This monograph offers a comprehensive and historically grounded examination of British-Irish political relations, from the Anglo-Norman invasion of Ireland in 1169 to the post-Brexit environment of the early twenty-first century. While the historical scope of the study is extensive, particular analytical emphasis is placed on the modern and contemporary periods—specifically, from the Anglo-Irish Treaty of 1921 to the United Kingdom’s withdrawal from the European Union between 2016 and 2020, and the subsequent political, legal, and diplomatic challenges that emerged between 2020 and 2025.

The core objective of the study is to trace the transformation of British-Irish political interactions within evolving frameworks of sovereignty, national identity, intergovernmental cooperation, and regional integration. At the heart of this analysis lies the argument that historical legacies remain central to shaping Anglo-Irish relations, yet contemporary shifts in international political architecture—most notably Brexit—have introduced new pressures and variables that fundamentally challenge the institutional resilience and diplomatic assumptions that had governed relations since the peace process of the late twentieth century.

The methodological approach adopted in this work is predominantly qualitative and historical-analytical. It is rooted in the systematic examination of a wide range of primary sources, including constitutional texts, parliamentary debates, intergovernmental treaties, legal judgments, official correspondence, and policy documents. These sources are interpreted through critical engagement with an interdisciplinary body of secondary literature encompassing political history, comparative politics, international relations, and legal scholarship. This interdisciplinary approach allows for a multifaceted understanding of the political evolution and institutional dynamics that have shaped the bilateral relationship between Ireland and the United Kingdom.

Particular attention is given to conceptual frameworks drawn from institutional theory, including path dependency and institutional evolution, which are vital for understanding the long-term consequences of political settlements such as the Good Friday Agreement. Constructivist approaches are also employed to examine how evolving conceptions of sovereignty and national identity have influenced political discourse and policymaking, especially in the context of competing unionist and nationalist narratives. In addition, this study uses comparative methods to analyze the divergent ways in which Ireland and the United Kingdom have responded to major geopolitical developments, such as European integration, the global financial crisis, and Brexit.

Of special significance is the role of the European Union, which since 1973 has served not only as an economic and political anchor but also as a structural framework within which British-Irish cooperation has taken place. The EU’s influence in facilitating peaceful coexistence and shared regulation has been substantial, and its loss as a common framework following Brexit constitutes one of the most consequential changes to the bilateral relationship in recent history.

To guide the interpretive structure of this analysis, four interrelated scientific hypotheses are proposed.

1. The first hypothesis posits that the United Kingdom’s withdrawal from the European Union has significantly disrupted the institutional architecture of British-Irish relations, weakening key mechanisms of cooperation established under the Good Friday Agreement and contributing to greater political and constitutional instability in Northern Ireland.
2. The second hypothesis contends that despite the dislocations caused by Brexit, new opportunities for pragmatic engagement and diplomatic recalibration have emerged, particularly as both governments seek to preserve peace in Northern Ireland and safeguard economic stability across the islands.
3. The third hypothesis addresses the enduring influence of historical narratives and contested identities, arguing that these symbolic and emotive dimensions continue to inform political decision-making and often obstruct technocratic or legalistic solutions, particularly in relation to the constitutional status of Northern Ireland.
4. The fourth hypothesis introduces the notion of growing asymmetry in diplomatic capacity and strategic positioning between Ireland and the United Kingdom in the post-Brexit period. While Ireland has increasingly relied on its EU membership and multilateral alliances to assert its influence and safeguard its interests, the UK’s pursuit of unilateral policy paths has contributed to increased legal friction and institutional strain, as evidenced by the contentious passage of the Legacy and Reconciliation Bill and subsequent legal challenges initiated by the Irish government.

Although the book engages with developments from the medieval period through to the twenty-first century, its analytical focus lies primarily in the twentieth and twenty-first centuries. Historical overviews are included where necessary to illuminate structural continuities and ideological legacies, but the central lens remains political and diplomatic. The study is not intended to serve as an exhaustive chronological history, but rather as a thematically and analytically focused investigation into the evolution of bilateral relations and their intersection with broader questions of identity, sovereignty, and peacebuilding. Cultural and economic dimensions are considered where relevant, but they remain subordinate to the central concern with political architecture, legal frameworks, and intergovernmental diplomacy. The scope of the work is necessarily shaped by the unfolding nature of many of the processes under consideration—particularly Brexit and its aftershocks—which remain dynamic and, in some cases, unresolved. Nevertheless, by situating these recent developments within a deeper historical and theoretical context, the study contributes to a broader understanding of post-Brexit Europe, the future of divided societies, and the challenges of maintaining cooperative statecraft in an increasingly fragmented international system. The structure of the book’s content is organized into four main chapters, along with an introduction and a conclusion.

Chapter One: Anglo-Irish Relations Before Ireland’s Independence (1169–1921)

This foundational chapter provides a longue durée overview of the structural inequalities and political subjugation that defined early Anglo-Irish relations. Section 1.1 offers a two-phase analysis of Anglo-Irish relations from the Anglo-Norman invasion to the Act of Union, focusing on political domination, the Tudor conquest, and the imposition of religious uniformity under the Act of Supremacy. Section 1.2 examines Ireland’s integration into the United Kingdom post-1801, with emphasis on Catholic emancipation, the rise of Irish nationalism, and the turbulent pathway to independence marked by the Easter Rising and the Anglo-Irish Treaty of 1921.

Chapter Two: Ireland and UK Political Relations from Anglo-Irish Treaty to EEC Accession (1921–1972)

This chapter addresses the evolving nature of bilateral diplomacy in the inter-war and post-war periods. Section 2.1 explores the contested legitimacy of the Treaty settlement, the rise of Fianna Fáil, Ireland’s neutrality during WWII, and the long-term impact of constitutional change culminating in the Republic’s proclamation in 1949. Section 2.2 assesses post-war economic and political interactions, emphasizing how both states transitioned from imperial legacies to modern sovereignty, and traces the early roots of European integration as a shared strategic goal.

Chapter Three: United Kingdom and Ireland, European Union, and Northern Ireland Peace Process (1973–2016)

This chapter focuses on the transformative influence of the European Community/EU and the overlapping trajectory of the Northern Ireland peace process. Sections 3.1 and 3.2 examine how EU membership fostered convergence and institutional dialogue. Sections 3.3 and 3.4 shift focus to the conflict in Northern Ireland, the multiparty negotiations of the 1990s, and the implementation of the Good Friday Agreement, underscoring the EU’s role in facilitating cross-border cooperation, legal harmonization, and socio-political reconciliation.

Chapter Four: Brexit and British-Irish Political and Economic Relations after Brexit (2016–2025)

The final analytical chapter explores the deeply disruptive and still-evolving effects of Brexit. Section 4.1 analyses diverging public attitudes toward European integration in the UK and Ireland, while Section 4.2 investigates the complexities of the Brexit process itself, especially the Northern Ireland Protocol. Section 4.3 discusses the renewed constitutional debate around Irish unity and the political implications for Northern Ireland. Section 4.4 evaluates both the deterioration and redefinition of British-Irish diplomatic relations in the context of ongoing negotiations, border politics, and legal disputes.

An integral part of this study is a critical engagement with the relevant scholarly literature, which has profoundly informed the analytical framework and interpretive strategies of this monograph. The work of J. Bowyer Bell is particularly valuable in understanding the historical dimensions of political violence and the organizational dynamics of the Irish Republican movement. His in-depth studies of the IRA highlight the legacy of colonial subjugation and the emergence of militant nationalism as a response to institutional exclusion and cultural suppression. Tim Pat Coogan, one of the most prominent chroniclers of modern Irish history, provides a comprehensive narrative of Irish nationalism and the Northern Ireland conflict. His biographies and historical overviews, although occasionally marked by a nationalist perspective, offer significant insight into the ideological foundations of republican politics and the complexities of post-colonial state formation in Ireland, especially under the leadership of Éamon de Valera. Etain Tannam’s contributions bring a vital theoretical dimension to the study of post-agreement British-Irish relations. Her work on intergovernmental cooperation and institutional design within the context of the Good Friday Agreement underscores the importance of structured, rules-based diplomacy in maintaining peace and stability. Tannam’s analyses are particularly pertinent in assessing the institutional vulnerabilities exposed by Brexit and the declining capacity of pre-existing frameworks to manage cross-border disputes in its aftermath. Jon Tonge and Jennifer Todd provide essential perspectives on political identity, party transformation, and post-conflict governance in Northern Ireland. Tonge’s research into the evolution of unionist and nationalist party platforms, especially within the Democratic Unionist Party (DUP) and Sinn Féin, offers a nuanced view of shifting political allegiances and the fragility of the power-sharing arrangements. Todd, by contrast, offers a more sociological and constructivist lens, exploring how symbolic boundaries, emotional attachments, and identity discourses shape conflict and reconciliation. Her work is instrumental in understanding the persistent cultural and ideological undercurrents that challenge technocratic and legalistic approaches to peacebuilding.

Together, these scholars and the others (e.g., Diarmuid Ferriter) form an interdisciplinary corpus that enriches the analysis of British-Irish relations by incorporating historical, political, legal, and sociological perspectives. Their research facilitates a deeper understanding of how the interplay of institutions, identities, and geopolitical shifts continues to shape one of Europe’s most complex and historically charged bilateral relationships.1 Context of history is introduced where necessary to clarify enduring structural patterns and ideological legacies, but the primary focus of the study remains political and diplomatic. Rather than offering a comprehensive chronological account, the analysis adopts a thematic and analytical approach to examine the evolution of bilateral relations and their intersection with broader issues of identity, sovereignty, and peacebuilding. Cultural and economic factors are addressed where they bear directly on these concerns, but they remain secondary to the core emphasis on political institutions, legal frameworks, and intergovernmental diplomacy. The scope of the study is inevitably shaped by the ongoing and unresolved nature of several key processes—most notably Brexit and its aftermath—which continue to evolve. By situating these contemporary developments within a deeper historical and theoretical framework, the study seeks to contribute to a wider understanding of post-Brexit Europe, the governance of divided societies, and the challenges of sustaining cooperative statecraft in an increasingly fragmented international order.

The concluding section synthesizes the study’s findings in light of the four guiding hypotheses. It argues that the future of British-Irish relations will hinge on the capacity of both states to adapt institutional mechanisms and diplomatic practices to a post-Brexit order, while managing the historical weight of identity, sovereignty, and contested borders. In doing so, it underscores the need for continued engagement with supranational frameworks, legal cooperation, and peacebuilding paradigms.