Is Keenan a first name or a last name?

Keenan is both: a family name in some places and a given name in others. It is mostly a family name in Canada, the United Kingdom and Ireland.

How to address someone called Keenan

United States. Keenan is used both ways: about 20,353 people had it as a surname in the 2010 census, and 16,866 babies born between 1950 and 2017 were given it as a first name. Look at where it sits in the name and how the person signs.
Canada. Keenan is a family name, usually written last. Write Dear Mr. Keenan or Dear Ms. Keenan.
United Kingdom. Keenan is a family name, usually written last. Write Dear Mr. Keenan or Dear Ms. Keenan.
Ireland. Keenan is a family name, usually written last. Write Dear Mr. Keenan or Dear Ms. Keenan.

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 Keenan as a family name or as a given name, by country of citizenship (145 in total).

Country	As family name	As given name	Family name share
United States	51	26	66%
Canada	16	3	84%
United Kingdom	14	1	93%
Ireland	14	0	100%
Australia	5	1	83%
New Zealand	4	0	100%
South Africa	1	2	33%

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

As a surname: about 20,353 people in the 2010 census, rank 1,765 among US surnames.
As a first name: 16,866 babies born between 1950 and 2017, 2% of them girls.