Cognomen O'Callaghan (O'Callaghan 
, О'Каллаган 
) , throughout the world, is a common surname. The last name O'Callaghan is habitual in Ireland, where it is quite a common surname, Australia, and Namibia, where it is quite a rare surname. Not in comparison with a country's population, this name is the most popular in Ireland, Australia, and the United Kingdom.
| VARIANT | COUNTRY |
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| O'Callaghan | Canada, Australia, United States, Ireland, United Kingdom |
| О'Каллаган | Ukraine |
Callaghan, Ò'Callaghan, and Каллаган
| cornelius o'callaghan Irish politician (1681–1742) (b. 1680) |
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| cornelius o'callaghan (1693-1741) (b. 1693) |
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| robert o'callaghan Irish politician (b. 1710) |
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| cornelius o'callaghan Irish politician (1712 – 1781) (b. 1712) |
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| carlos o'callaghan (1730-1774) (b. 1730) |
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| dionisio o'callaghan (born 1732) (b. 1732) |
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| elena o'callaghan (1738-1775) (b. 1738) |
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| cornelius o'callaghan Irish politician (b. 1741) |
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| james o'callaghan politician (b. 1743) |
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edmund bailey o'callaghan Irish physician and journalist, IE (b. 1797) |
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| john o'callaghan (1754-1818) (b. 1754) |
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john cornelius o'callaghan Irish historian and writer, IE (b. 1805) |
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| ramon o'callaghan (1765-1833) (b. 1765) |
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| mary o'callaghan (b. 1756) |
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| maria ana o'callaghan (1766-1830) (b. 1766) |
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