John O'Hart

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John O'Hart (Irish: Seán Ó hAirt; 1824–1902) was an Irish historian and genealogist. He is noted for his work on ancient Irish lineage.

He was born in Crossmolina, County Mayo, Ireland. A committed Roman Catholic and Irish nationalist, O'Hart had originally planned to become a priest but instead spent two years as a police officer. He was an Associate in Arts at the Queen's University, Belfast. He worked at the Commissioners of National Education during the years of the Great Famine. He worked as a genealogist and took an interest in Irish history. He died in 1902 in Clontarf near Dublin, at the age of 78.

O'Hart's 800-page, The Irish and Anglo-Irish landed gentry (Dublin 1884), was reprinted in 1969, with an introduction by Edward MacLysaght, the first Chief Herald of Ireland. Another work, Irish pedigrees; or, The origin and stem of the Irish nation, first published in 1876, has come out in several subsequent editions.^{[citation needed]}

To complete his genealogies he used the writings of Cú Choigcríche Ó Cléirigh, Dubhaltach Mac Fhirbhisigh and O'Farrell, along with the Annals of the Four Masters, for the medieval pedigrees. He used the works of Bernard Burke, John Collins and others to extend his genealogies past the 17th century.^{[citation needed]}