"Would-be criminals and terrorists are policed by services that cooperate as intimately as if they served only one sovereign. Britain and Ireland have found a way to be one marketplace, one travel area, one energy grid, one food emporium, one security partnership—while maintaining two independent political systems"
Brexit Could Reawaken Northern Ireland’s Troubles - The Atlantic
https://www.theatlantic.com/ideas/archive/2019/03/brexit-could-reawaken-northern-irelands-troubles/584338/