You don't disagree with me - you disagree with internationally recognised geographical realities.
Republic Of Ireland is a country. Northern Ireland is part of another country, the UK. Ireland therefore is not one entity.
That's about as disputable as the fact that the English term for the colour of the sky is "blue".
I feel like I should probably deconstruct the ignorance of this rather than just pointing it out all the time.
Look at it this way. Just say, someone from Sweden sailed over to the North of England, planted down a flag and declared that six counties up there were now a country of their own, "Northern England", and were subsidiary to Sweden. Would England cease to be a country?
What if they came with enough military firepower to enforce this point of view on the whole of the British Isles. Would it then become a country? What if it waited fifty years? Would it at some stage suddenly transform into a country of its own?
Just say a very marginal majority in Northern England wanted to be part of Sweden. Would England, as an entity, then immediately cease to exist?
At what point would a country become no longer a country? These are questions everyone in Ireland had to ask themselves in 1921. The concept of cutting a country in two was (extremely) repulsive both to Unionists and Republicans, but was reluctantly accepted as the only realistic compromise.
What would be done with the England sporting teams? Would they be forced to divide themselves between those in the North and those in the South? Or would they continue on as though nothing has happened? If you were asked, at that point, whether you considered yourself Northern English or Southern English, what would your response be? If you said you just considered yourself English, how would you react to being told that such a country, and hence such a nationality, does not exist?
Just say there were 75 years of war over the issue, with the English sporting sides opting to remain intact. How ignorant would someone suggesting that they should all play together under the single banner of "Sweden and England" sound?