writer vs O'Brien

writer and O'Brien both are nouns.

Word	Noun	Adjective	Verb	Adverb
writer	Yes	No	No	No
O'Brien	Yes	No	No	No

writer (noun)	O'Brien (noun)
writes (books or stories or articles or the like) professionally (for pay)	Irish writer (born in 1932)
a person who is able to write and has written something