Much of the cooperation on criminal justice matters between the United Kingdom and the Republic of Ireland is based on EU level instruments. While there has been consideration of the broader impact of Brexit on the Good Friday Agreement and consensus on the need to avoid a return to a hard border between Ireland and Northern Ireland, more detailed consideration has not been given to the effect that Brexit may have on continued criminal justice cooperation across the border. This article highlights the combined risks that Brexit presents for Northern Ireland in the form of increased criminality at a time when the loss of EU police cooperation mechanisms may result in a reduction of operational capacity and the removal of the legal architecture underpinning informal cooperation. Part 1 seeks to highlight the historical context of UK-Irish cooperation in policing matters. Part 2 explores the risk that post Brexit the Irish border may become a focus for criminal activity. The risks relating to increased immigration crime, smuggling of commodities and potential rise in terrorist activities are explored. Part 3 considers how the risks of increased criminal threats are exacerbated by the loss of EU criminal justice cooperation mechanisms and how this will affect UK-Irish cooperation specifically. Consideration is particularly given to the loss of information sharing systems. Part 4 considers how loss of EU level cooperation mechanisms could be mitigated. The viability of bilateral agreements between the UK and Ireland is considered alongside ways which police cooperation can be formalised to compensate for the potential loss of EU criminal justice information sharing systems. Nordic police cooperation is considered as a potential blueprint for the UK and Ireland.