It seems that essentially what you're saying is that a Nation is not it's people:
- that it would, and should, be possible to completely replace the population of (say) Japan with Irish people, and as long as they respect Japanese culture, language, tradition, religion, etc.. more than the Irish equivalents, it will still recognizably be Japan?
- or that say, if millions of Africans, Arabs, and Chinese decide they would like to move to Ireland, they should be able to as long as they are law-abiding, well-mannered, and respect Irish culture - even if the Ethnic Irish population becomes only 15% of the population of the homeland their people have lived in for thousands of years?
Does the land and entity that is called Turkey belong to the Turks? Do they have a right to consider it their homeland? Do they have a right as a people to decide if they should be a majority or minority within Turkey?
>"I think what you fear is a possible loss of cultural identity."
That's one part of it, sure, but only one part. Ultimately this is about possession. Ireland belongs to the Irish. The land is theirs. Poland belongs to the Poles. Those Ethnic groups should be able to decide for themselves if they want to give it away.
(unless you take the Communist view that no one really owns anything/property is evil, immoral?)
In other words, this open-ended, un-limited premise that "Any Ethnic Group should be able to move anywhere, in any number, as long as they behave themselves" is a licence for a slow-motion replacement of Ethnic Groups within their homelands, and for the theft of something that belongs to their people.
When this happens quickly and violently, we call this an Invasion. If it's spread out over a few decades - even if it happens against the wishes of the majority population - this is called merely "migration" - even if the end result is the same - replacement, displacement, and dispossession)