O'Leary is an Irish name, an anglicized version of the original Gaelic patronym Ó Laoghaire (IPA:oːˈl̪ˠiːəɾʲə) or Ó Laoire.
The Uí Laoghaire clan, today associated with the Uibh Laoghaire parish in County Cork, is considered by scholars to have originated in the early Middle Ages on the south-west coast, in the area of Ros Ó gCairbre (Rosscarbery), of which the O'Leary were hereditary lords.
The Annals of Innisfallen (Dublin copy) records St. Fachtna's (Saint Fachanan) death in 600 AD as occurring in "O'Laeghaire of Ross i.e. Corca Laidhe-I-Laeghaire Ruis". The clan traces its lineage to Lugaid Mac Con, an ancient King of Tara and High King of Ireland, and descendant of Dáire Doimthech. In the 12th century the O'Leary's were recognised hereditary wardens of St Fachtna's monastery and seat of higher learning, the School of Ross. In more recent times (since 1300 AD), the clan, of the Corcu Loígde, was pushed north and settled in an area south of Macroom around Inchigeelagh on the River Lee called Uibh Laoghaire (or Uibh Laoire in modern Irish – the 'gh' is silent in the old Irish). The Corcu Loígde were the rulers of Munster, and of territories beyond the province, before the rise of the Eóganachta in the 7th century. The port of Dún Laoghaire, near Dublin is not associated with the O'Leary sept(s), rather it is named for Lóegaire mac Néill, a 5th-century High King of Ireland.
The book of Lecan (A.D. 1397-A.D. 1418) details the early status of O'Leary as a Corco Laide taisach duchusa (hereditary lord) in the tuath of Ross (Ruis), with associated families:
Tuath Ruis .i. Tuath in Dolaich, o Loch in Bricin co Faid Ruis -, o Thraig Long co Sid na Fear (i) Find. O Leagaire a taisach duchusa. Is iad so an oclaid duchusa .i. O Ruaidri -, O Lonan -, O Laidid -, O Torpa -, O hUrmoltaich -, O Mirin -, O Meic Dairic -, O Tuaraide -, O Trena -, O hUainidi -, O Cerdin 
The name also occurs in the Cineal Laoghaire branch of the Eoghanacht dynasty which later came to dominate Munster. With the unrelated Corco Laidhe and Eoghanacht branches of O'Learys settling in north-west Cork and nearby Kerry respectively, the tracing of lineage is complex.
Although almost nothing is known of their activities for several centuries, the O'Learys reappear as a still titled  family in the 16th century, and relatively wealthy, although they were subject to the MacCarthy of Muskerry dynasty, from whom they received the White Wand (a symbol of authority). They were the only other freeholders in Muskerry besides the O'Mahonys, and had built several castles in their territories, of which Carrignacurra is now the only one still standing. The celebrated Irish language writer Peadar Ua Laoghaire was a descendant of the Carrignacurra branch of the family.
Auliffe O'Leary joined the side of Hugh Ó Neill, 2nd Earl of Tyrone in the Nine Years' War, from the very inception of it, and took the field with William Bourke (Clanricarde) and others. For this the chiefs of the O'Learys were eventually attained, and their lands parceled out, but because of the remoteness of their territory it was never carried out, and they remained safe there until the Cromwellian confiscations decades later. Donough MacCarty, 1st Earl of Clancarty did however appear to do his best to allow them to stay on their lands through leasing. The family became much more scattered during the later Williamite War in Ireland.
As an example of their wealth and capacity in the mid-16th century, an early O'Leary of Carrignacurra  is purported to be the fosterer of Donnel na g croiceann, or Donnel of the Hides, ancestor of the modern O'Donovans of Clan Cahaill. His daughter Ellen married Donnell. Their issue was Donnell II O'Donovan.
The last O'Leary lord of the Old Gaelic order was Donal MacArt O'Leary (1575–1657).
- Art Ó Laoghaire (d 1773), Austro-Hungarian army officer
- Daniel Florencio O'Leary (1802–1854), military general under Simon Bolivar
- Michael John O'Leary (1890–1961), Irish-Canadian recipient of the Victoria Cross
- Arthur O'Leary (1729–1802), Irish Franciscan and polemical writer
- Francis O'Leary MBE (1931–2000), Roman Catholic priest and missionary who founded the St Joseph's Hospice Association
- Henry Joseph O'Leary (1879–1938), 5th Bishop of the Roman Catholic Diocese of Charlottetown and 3rd Archbishop of the Archdiocese of Edmonton
- Louis James O'Leary (1877–1930), 6th Bishop of the Roman Catholic Diocese of Charlottetown
Poetry and literature
- John O'Leary (Fenian) (1830–1907), Irish poet who was imprisoned in England during the nineteenth century
- Máire Bhuí Ní Laoghaire (1774–1849), Irish poet
- Peadar Ua Laoghaire (1839–1920), Irish writer and Catholic priest, regarded today as one of the founders of modern literature in Irish
Politics and law
- Brendan O'Leary, Irish political scientist
- Clement O'Leary, Canadian member of Parliament
- Cornelius O'Leary, Irish historian and political scientist
- Denis O'Leary (1863–1943), New York politician
- Grattan O'Leary (1888–1976), Canadian journalist and Senator
- Hazel R. O'Leary (born 1937), former United States Secretary of Energy
- Henry O'Leary, Irish-born businessman and political figure in New Brunswick
- Humphrey O'Leary (1886–1953), 7th Chief Justice of New Zealand
- James A. O'Leary, member of the United States House of Representatives from New York
- Jean O'Leary, American gay and lesbian rights activist, politician, and former nun
- John O'Leary (ambassador) (1947–2005), mayor of Portland, Maine, and United States ambassador to Chile
- John O'Leary (Kerry politician) (born 1933), former Irish Fianna Fáil party politician and TD for Kerry South
- John O'Leary (Wexford politician) (1894–1959), former Irish Labour party politician and TD for Wexford
- Joseph V. O'Leary, NYS Comptroller 1941–1942 and founding member of the Liberal Party of New York
- Kevin O'Leary (judge) (born 1920), 2nd Chief Justice of the Supreme Court of the Northern Territory
- Michael O'Leary (politician) (1936–2006), former leader of the Irish Labour Party
- Seán O'Leary (Irish politician) (born 1941), former Irish senator
- William O'Leary (Irish politician) (died 1955), Irish Fianna Fáil politician and TD for Kerry
Journalism and activism
- Émile-Dostaler O'Leary (1908–1965), Canadian journalist and writer
- Grattan O'Leary (1888–1976), Canadian journalist and Senator
- Jeremiah O'Leary, American journalist
- John O'Leary (journalist), editor of the Times Higher Education Supplement
- Olivia O'Leary, Irish journalist
- Walter-Patrice O'Leary (1910–1989), Canadian journalist, political activist and trade unionist
Arts and entertainment
- Arthur O'Leary (1834–1919), Irish composer and pianist
- Dermot O'Leary (born 1973), English television and radio presenter
- Fletcher O'Leary, Australian child actor
- Jane O'Leary (born 1946), American-Irish avantgarde composer
- Mary O'Leary, American television producer
- Matt O'Leary, American actor
- Michael O'Leary (actor) (born 1958), actor in the long-running American soap opera Guiding Light
- Patrick O'Leary (writer) (born 1952), American science fiction writer
- Tim O'Leary, fictional character in the British soap opera Brookside
- William O'Leary (actor), American actor
- Amanda O'Leary, American lacrosse coach
- Bob O'Leary, American soccer player
- Charley O'Leary (1882–1941), American baseball player
- Clare O'Leary, first Irish woman to climb Mount Everest
- Dan O'Leary, American baseball player
- Daren O'Leary, English rugby union player
- David O'Leary, English-born Irish football (soccer) manager formerly of Aston Villa
- George O'Leary (born 1946), American college football coach
- John O'Leary (Gaelic footballer)
- Kieran O'Leary, Irish Gaelic footballer
- Kristian O'Leary, Welsh football (soccer) player
- Noel O'Leary, Irish footballer
- Peter O'Leary, New Zealand football (soccer) referee
- Pierce O'Leary, Irish footballer
- Ryan O'Leary, Scottish footballer
- Seánie O'Leary (born 1954), Irish hurler
- Stephen O'Leary, English-born Irish footballer
- Tomás O'Leary (born 1983), Irish rugby union player
- Troy O'Leary (born 1969), American baseball player
- Brian O'Leary, American "scientist-astronaut"
- Catherine O'Leary (1827–1895), owner of the property in which it was alleged the Great Chicago Fire started
- Ciarán O'Leary, Irish professional poker player practicing in the USA
- Daniel O'Leary (mobster), Irish American mobster
- James Patrick O'Leary (c. 1860–1926), American mobster
- Kevin O'Leary (entrepreneur), Canadian entrepreneur and venture capitalist
- Michael O'Leary (Ryanair), CEO of the low-cost Irish airline Ryanair
- Timothy PJ O'Leary, (1985-present) "I shave every now and then."
- Dún Laoghaire, Dublin, Ireland – Name means "Fort of Leary".
- O'Leary, Prince Edward Island
- Plaza O’Leary – major square in Caracas, Venezuela. Named for Daniel Florencio O'Leary
- Dublin Annals of Inisfallen
- John O'Donovan, ed. 'The Genealogy of Corca Laidhe', in Miscellany of the Celtic Society. Dublin. 1849. alternative scan
- Cork History and Society, P. O'Flanagan, C. Buttimer, Geography Publications, Dublin 1993
- John O'Donovan
- Cork History and Society, P. O'Flanagan, C. Buttimer, eds. Geography Publications, Dublin 1993
- Cork History and Society, P. O'Flanagan, C. Buttimer, eds. Geography Publications, Dublin 1993, p 63
- In the Gaelic system.
- Ó Murchadha, p. 208
- Diarmuid Ó Murchadha, Family Names of County Cork. Cork: The Collins Press. 2nd edition, 1996. pp. 206 ff
- Ó Murchadha, p. 214
- Ó Murchadha, p. 211
- The branch to which this O'Leary belonged is not precisely known. It may have been different from the one that later occupied it.
- Donal MacArt O'Leary by Peter O'Leary