The big thing in both the North and the South is the border, and the troubles have a massive colouring job on thoughts about that issue, something that will make a great difference to expats on both sides of the border if the UK leaves the EU without a deal.
Nobody wants a border because it harks back to the violence, but there has to be a border (or reunification of North and South) if the UK goes no deal.
Basically, a border expats can cross without so much as a second glance at the moment will become a customs post, or there is likely to be a lot of violence, another big headache for expats.
That leaves expats on both sides of the border with a headache.