Is Brennan a first name or a last name?

Brennan is a family name (last name). It is mostly a family name in the United States, Ireland, the United Kingdom and Australia.

How to address someone called Brennan

United States. Brennan is a family name, usually written last. Write Dear Mr. Brennan or Dear Ms. Brennan.
Ireland. Brennan is a family name, usually written last. Write Dear Mr. Brennan or Dear Ms. Brennan.
United Kingdom. Brennan is a family name, usually written last. Write Dear Mr. Brennan or Dear Ms. Brennan.
Australia. Brennan is a family name, usually written last. Write Dear Mr. Brennan or Dear Ms. Brennan.

Not sure which part of a full name is which? Check the full name with the country where the person is based.

People recorded in Wikidata who carry Brennan as a family name or as a given name, by country of citizenship (437 in total).

Country	As family name	As given name	Family name share
United States	171	29	86%
Ireland	77	0	100%
United Kingdom	54	1	98%
Australia	53	1	98%
Canada	28	7	80%
New Zealand	5	1	83%
France	4	0	100%

Wikidata covers notable people, so counts show how the name is used, not how many people carry it.

As a surname: about 49,238 people in the 2010 census, rank 701 among US surnames.
As a first name: 33,095 babies born between 1950 and 2017, 6% of them girls.