Kavanagh is one of the very few ancient Gaelic Irish surnames which has neither the prefix Mac or O: it is wrong to call it O Caomhanach in Irish as is sometimes erroneously done. In Irish it is simply Caomhanach which is an adjective denoting assocation with Caomhan, in this case St. Caomhan, the first Kavanagh having been fostered by a successor of that saint. It was not customary for such epithets to be perpetuated, as happened with this branch of the MacMurroughs. The first Kavanagh was Donal son of Dermot MacMurrough, King of Leinster, who was one of the prominent figures in Irish history, being the immediate cause of the Anglo-Norman invasion. The Kavanagh territory lay then in Counties Wexford and Carlow and they contained to be extensive landowners there up to recent times. The name is very numerous in and around County Wexford in all classes of society, so much so indeed that there are enough Kavanaghs in the south-eastern counties of Leinster by themselves, without counting the scattered Kavanaghs in the rest of the country, to put the name in the list of eighty commonest surnames in the country: all told they hold fifty-third place in that list.

The agnomen Kavanagh was long associated with the MacMurroughs, Art Mac Murrough, the King of Leinster who put so determined a resistence to Richard II of England, being styled Kavanagh. The Kavanaghs themselves have produced a number of notable figures, none more picturesque than Arthur MacMurrough Kavanagh (1831-1889), who, although he had only stumps of arms and legs, overcame the disability and became an expert horseman and fisherman, learned to write and draw and was for many years a Member of Parliament. In the same century Morgan Peter Kavanaugh (1800-1874) and his daughter Julia Kavanagh (1824-1877) were well known authors in their day. Going back to the sixteenth century there was Cahir Mac Art Kavanagh (1500-1554), who took part in the Geraldine rebellion, and Art Kavanagh, who was Hugh O'Neill's companion in the dramatic escape from Dublin Castle in 1590. In the next century we find Brian Kavanagh, one of the many Kavanaghs who fought for the Stuart cause described as the tallest man in King James' army; while amoung the Wild Geese of the name Morgan Kavanagh, who rose to be Governor of Prague in 1766, was said to be the biggest man in Europe. Several Kavanaghs were officers in the Irish Brigade in the army of France and a branch of the family settled in that country, but it was in Austria they chiefly distinguished themselves. Two were prominent in 1798 - Rev. Francis Kavanagh, who was one of the leaders of the inssurection in County Wexford, and Walter Cavanagh of Borris, County Carlow, nicknamed by the people "the monarch", whose house was burned down by insurgents. The well-known song "Eileen Aroon", said to be composed by Carol O'Daly in the thirteenth century, should be mentioned in connexion with this family, the Eileen invoked being the daughter of the Kavanagh chief of the time.

Kavanagh is sometimes used as a synonym for two often quite distinct surnames, affording an example of the not uncommon process of attraction whereby some well known patronymic of somewhat similar sound is assumed in place of the original name. O Caomhain, anglice O'Keevan and Kevane, once as important sept in Mayo, where is has also been maladroitly turned into Cavendish, is one; the other is O Caibhdeanigh of Ossory, an obsolete form of Gaffney.