This paper provides the most comprehensive assessment to date of cross-border strategic spatial planning on the island of Ireland, examining the evolution of planning systems, the effectiveness of existing governance arrangements, and the implications of emerging policy divergence. Drawing on a mixed-methods approach comprising policy analysis, literature review, interviews with senior planners and policymakers, and an online survey, the study evaluates the extent of strategic alignment between Northern Ireland and Ireland and identifies opportunities and challenges for future collaboration.

The findings reveal that strategic spatial planning has become an increasingly important mechanism for shaping sustainable development across the island since the introduction of Northern Ireland’s Regional Development Strategy (2001) and Ireland’s National Spatial Strategy (2002), both influenced by European concepts of territorial cohesion and polycentric development. While local government reforms and cross-border networks have strengthened cooperation and created new “soft spaces” for engagement, significant structural barriers remain.

The research highlights a growing risk of strategic divergence. Ireland has modernised its planning framework through the National Planning Framework (2018; revised 2025), while Northern Ireland continues to rely on the Regional Development Strategy 2035, last updated in 2012. This widening temporal and policy gap threatens long-term spatial coherence across the island. The paper concludes that, while strong foundations for cross-border planning exist, addressing shared challenges will require a transition from cooperation towards deeper collaboration.