Hennessy (or 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:
Arthur Hennessy (1876–1959), Australian pioneer rugby league identityBill Hennessy, former Irish sportsmanCarly Smithson (née Hennessy, born 1983), Irish singerClaire Hennessy (born 1986), Irish novelistClinton Hennessy (born 1977), Irish sportspersonDavid Hennessy (1858–1890), police chief of New OrleansGeorge Hennessy, 1st Baron Windlesham (1877, 1953), British soldier and Conservative politicianJean Hennessy (1874-1944), French politician and member of the Hennessy familyJacqueline Hennessy (born 1968), journalistJill Hennessy (born 1968), film and television actressJoe Hennessy (born 1956), former Irish sportspersonArchbishop John Hennessy (1825–1900), former head of the Roman Catholic Archdiocese of DubuqueJohn L. Hennessy (born 1953), American computer scientist and current president of Stanford UniversityJohn Pope Hennessy (1834–1891), eighth governor of Hong KongJosh Hennessy (born 1985), professional ice hockey playerKevin Hennessy (born 1961), Irish retired sportspersonLarry Hennessy (1929–2008), American basketball playerMatthew Hennessy, Irish computer scientistMiranda Hennessy, British actress and comedianNeil Hennessy (born 1978), musician and music producer/engineerPatrick Hennessy, several people includingPatrick Hennessy (painter), (1915-1980), an Irish artistSir Patrick Hennessy, (1898–1981), an Irish-born British industrialist,Peter Hennessy (born 1947), English historian of governmentSéamus Hennessy, Irish sportspersonSwan Hennessy (1866–1929), Irish-American composer resident in ParisW. Louis Hennessy (born 1955), 4th Circuit Court Associate Judge in Maryland, former Maryland DelegateWilliam John Hennessy (1839–1917), Irish painter