This is very loosely based on recent rumors about Northern Ireland potentially remaining within the EEA or even EU while the rest of Britain exits (sadly the DUP isn't on board atm). Once that agreement is reached, the following things happen over the next 20 years:
-Many banks and businesses move from London or other cities on the island of Britain to Belfast and its suburbs (Antrim, Bangor, etc).
-There's a large wave of emigration from Britain proper to Northern Ireland associated with these businesses. 80-90% of the migrants holding British passports are Remainers. About 15% of the migrants are from elsewhere in the EU, particularly Poland.
-Northern Ireland sees Sunbelt-like population growth of 3-5% per year initially, tapering off after about 10 years but equating to a 50-60% increase in the resident population over 2019.
-Many of the British migrants do not embrace or take sides in the national divide, instead identifying as European. At the same time, Northern Irish identity increases significantly, particularly in sought-after coastal towns and scenic rural areas that are either too affluent to care about sectarianism or associate being "British" with gentrification.
-By 2039, the 'national identity' map of Norn Iron looks like this:
Nonsectarian identifies (in yellow) such as "Northern Irish" or "European" have grown substantially. A plurality of people in four districts (yellow) identify in a nonsectarian fashion.
The large-scale influx from mainland Britain has resulted in a significant increase in "British" identity (blue) in Belfast and its surrounding areas, but most of the "New British" are secular and apathetic to the region's divide, instead being lured to Northern Ireland by job opportunities (often in finance).
Nationalists/"Irish" (green) have continued to grow in population in the south and west as they had in prior censuses, partly due to a growing Roman Catholic population but partly also due to former unionists becoming disaffected with the influx of mainlanders turning Belfast into "Little London".