national capital vs capital of Northern Ireland

national capital and capital of Northern Ireland both are nouns.

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

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