There are TWO continents called America - one is South America and one is North America. The nouns 'South' and 'North' are added to the names of these continents so that we can distinguish between them. There is ONE island called Ireland. The adjective 'northern' refers to northerly part of a place. The term 'Northern Ireland' is used to denote the top-right corner of the island, which is part of the United Kingdom. This term is used to distinguish it from the rest of the island, which is a country in its own right: the Republic of Ireland, or Eire.

As usual, Su. Ki. has given a correct, clear answer to the question.