Belfast vs capital of Northern Ireland

Belfast and capital of Northern Ireland both are nouns.

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

As nouns, Belfast and capital of Northern Ireland are synonyms defined as:

Belfast and capital of Northern Ireland: capital and largest city of Northern Ireland; the center of Protestantism in Northern Ireland

Belfast (noun)	capital of Northern Ireland (noun)
capital and largest city of Northern Ireland; the center of Protestantism in Northern Ireland	capital and largest city of Northern Ireland; the center of Protestantism in Northern Ireland