Ireland and the United Kingdom: A Century of Relations (1921-2021) offers a comprehensive analysis of one hundred years of complex political, legal, and social relations between Ireland and the United Kingdom. Beginning with the partition of Ireland and the establishment of the Irish Free State, the book traces the evolving nature of bilateral relations through conflict, cooperation, and reconciliation. Particular attention is paid to constitutional developments, questions of sovereignty and identity, the Northern Ireland conflict, and the impact of European integration, including Brexit. Combining historical inquiry with legal and political analysis, the monograph adopts a multidisciplinary approach to capture the changing dynamics between the two states. By situating key events within their broader international and European contexts, the book provides a nuanced assessment of how past legacies continue to shape present and future relations between Ireland and the United Kingdom.