capital of Northern Ireland vs national capital

capital of Northern Ireland and national capital both are nouns.

Word	Noun	Adjective	Verb	Adverb
capital of Northern Ireland	Yes	No	No	No
national capital	Yes	No	No	No

capital of Northern Ireland (noun)	national capital (noun)
capital and largest city of Northern Ireland; the center of Protestantism in Northern Ireland	the capital city of a nation