© 2020, Geographical Society of Ireland. All rights reserved. Brexit poses major institutional and governance challenges for the island of Ireland, not least in the area of Marine Spatial Planning (MSP) and the cooperative governance and integrated management of shared marine spaces and ecosystems. To date, MSP scholarship has not delved into the complex processes that construct marine borders and has failed to acknowledge how the same border may have different boundary qualities across a range of institutional contexts. Using the case of transboundary marine governance on the island of Ireland, we evaluate recent marine governance innovations in the Republic of Ireland (ROI) and Northern Ireland (NI) to assess if they promote integration. We then illustrate how the boundary characteristics of marine borders are context dependent, with the same border being both hard and soft in differing contexts, and we discuss how this may be impacted by Brexit. We argue that the development of integrated marine management is not a priority in these jurisdictions and that the resulting complex, multiple constructions of marine borders has largely been ignored by MSP researchers.1 We conclude that developing a more nuanced understanding of borders in order to advance integrated marine management is crucial for post-Brexit MSP planning practice and research.