Hennessey is an Irish surname, being the anglicised form of Ó hAonghusa. Families of this name were found in Kingdom of Uí Failghe and Kingdom of Desmond.

Notable people with the surname include:

• Brad Hennessey (born 1980), baseball player
• Charlotte Hennessey (1873–1928), Canadian actress; mother of Mary Pickford (Gladys Louise Smith), Lottie Pickford and Jack Pickford
• Dorothy Hennessey (1913–2008), Roman Catholic religious sister and activist
• Gwen Hennessey (born 1932), Roman Catholic religious sister and activist
• John Hennessey, founder of Hennessey Performance Engineering
• Keith Hennessey, director of the U.S. National Economic Council
• Terry Hennessey (born 1942), Welsh footballer
• Wayne Hennessey (born 1987), Welsh footballer

• Alistair Hennessey, fictional rival to the eponymous character of The Life Aquatic with Steve Zissou

• Hennessy (surname)