Drennan is a surname of Irish origin and is derived from the Gaelic Ó Draighnáin, meaning "grandson of blackthorn". Notable people with the surname include:
Alexander Drennan (1899–1971), New Zealand labourer, trade unionist, communist and watersider
Anthony Drennan, Irish guitarist
Bruce Drennan (born 1950), American sports announcer
Bryce W. Drennan (born 1963), American writer
Charles Drennan (born 1960), New Zealand Romanc Catholic bishop
John Drennan, Irish journalist and writer
John Cherry Drennan (1899–?), Northern Ireland politician
John Swanwick Drennan, Irish poet
Kathryn M. Drennan, American writer
Mandy Drennan (born 1988), Australian Paralympic swimmer
Martin Drennan (born 1944), Irish Roman Catholic bishop
Michael Drennan known as 'Mikey' (born 1994) Irish professional footballer
Robert Drennan, American archaeologist
Thomas Drennan (1696–1768), Irish Presbyterian minister
Thomas J. Drennan (1877–1928), New York City Fire Commissioner
William Drennan (1754–1820), Irish poet
