I just woner (and perhaps someone can put my mind at rest) how they ACTUALLY calculate the number of people of any nationality in a country?
Through census returns. In Ireland they take a census every four years. It could be done through electoral rolls as well as all EU citizens can vote in local elections and your nationality is recorded but it's less reliable than the census as not everyone bothers to go on the electoral roll whilst most people complete the census forms. According to the 2011 Census, Polish nationals increased by 93.7 per cent since 2006 from 63,276 to 122,585 in 2011 making them the largest group ahead of UK nationals with 112,259.
naturalised. Is that person counted as Irish, British or Polish?
That's a very interesting question. You can declare two nationalities and I imagine that most people would like to be recorded as both Polish and Irish. Also your place of birth is recorded on the census return so with those two pieces of information it's possible for the Central Statistics Office to get a clear picture of the number of non-native Irish citizens there are. Also the Irish Immigration Service keeps records of all citizenships granted so it's an easy matter to determine how many Irish citizens were born in Poland. There's so few that it would hardly make much impact on the overall stats.