The O'Leary One-Name Study was initiated in 2023 to research all occurrences of the name, from its origins in Ireland to the United Kingdom and across the world. Information collected and published here will include indexes of births, marriages, and deaths by location; church records such as baptisms and confirmations; biographies; public family trees; published information including newspaper articles and academic papers; professional and military service records; photographs; DNA studies; and migration patterns/geographic distributions of the O'Leary name and it's variants.

Leary, O'Laoghaire, O'Laohaire, O'Laoire

The O'Leary name originated in southwest Ireland more than 1,500 years ago. It is an anglicised form of the old Gaelic name O'Laoghaire, where the prefix "O" signifies a male descendent. "Laoghaire" combines the Gaelic words for "calf" ("laogh") and "keeper" ("aire"). Hence, the literal meaning of the name: descendant(s) of the keeper of the calves.

The ancestry of the O'Leary's may be traced back to the High King of Ireland in the late 2nd century, Lugaid Mac Con. Descendants of Mac Con also include the Coffey, Flynn, Hennessy and O'Driscols of County Cork. The O'Leary sept were the hereditary wardens of St Factna's monastery in Rosscarbery until they settled in an area south of Macroom called Inchigeelagh where they ruled as chiefs under the MacCarthy dynasty of Muskerry.

The motto often seen on the O'Leary coat of arms is "Laidir isé lear Righ" -- Strong is the King of the sea.

The port town of Dún Laoghaire ("fort of Laoghaire") in County Dublin was named after the 5th century King of Ireland, Laoghaire Mac Néill who reigned from 428-458 CE in the time of St Patrick. A 1686 chart of Dublin Bay by Greenvill Collins, a Royal Navy officer and hydrographer to Charles II, shows the port as "Dun Lerroy".

Famous name bearers include priest and writer Fr Peadar Ua Laoghaire (1839-1920) who campaigned for land reform and preservation of the Irish language. Máire Bhuí Ní Laoire (1774-1848) was a poet whose work was passed on for generations purely by oral tradition, sung or recited. Daniel Florence O'Leary (1801-1854) was a military general and aide-de-campe under the Venezuelan political leader Simón Bolivar. John O'Leary (1830-1907) was a leading member of the Irish Republican Brotherhood, who was imprisoned for five years in England after being charged with high treason.