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Irish: Anglicized form of Gaelic Ó Catharnaigh ‘descendant of Catharnach’, a byname meaning ‘warlike’ or ‘soldier’, and Ó Cearnaigh, from cearnach ‘victorious’. Both surnames were widely distributed and are now very difficult to disentangle.

(Dictionary of American Family Names, Oxford University Press)
http://www.ancestry.com/search/SurnamePage.aspx?sourcecode=13304&html=b&fn=&ln=kearney&submit.x=0&submit.y=0