Britain vs Northern Ireland

Britain and Northern Ireland both are nouns.

Word	Noun	Adjective	Verb	Adverb
Britain	Yes	No	No	No
Northern Ireland	Yes	No	No	No

Britain (noun)	Northern Ireland (noun)
a monarchy in northwestern Europe occupying most of the British Isles; divided into England and Scotland and Wales and Northern Ireland; `Great Britain' is often used loosely to refer to the United Kingdom	a division of the United Kingdom located on the northern part of the island of Ireland