Heh. It's obviously two separate countries- there's an international border separating them. And the two countries are just as clearly, two states.
Even Sinn Fein have largely dropped that foreign occupation nonsense. Northern Ireland is populated almost entirely by Irish people, the vast majority of whom can trace their Irish ancestry back for centuries.
Their majority is manufactured only in the sense that they're both numerous (unionists have been ca 20% of the population of Ireland since the 19th century) and localised (in the area that's now NI). If the border was drawn differently- down the river Foyle say, with only Waterside in NI) the nationalist minority would be smaller than now.
Let's see your evidence for nationalists increasing to 51% and beyond. In the 2007 NIA election, it was 41.4% (SF 26.2%, SDLP 15.2%). To force the end of partition you'd need pretty much all the Alliance and Green voters, plus health campaigners and even a few unionists, to change sides. It isn't going to happen.
http://news.bbc.co.uk/1/shared/vote2.../html/main.stm