The Kavanaghs have a royal lineage and some of them have had distinguished if rumbustious careers. Diarmiud MacMurrough was the 12th-century King of Leinster. He was notorious for inviting the Anglo-Norman invasion of Ireland. His son Donal, in order to distinguish him from other Donals when he was being educated in Wexford, was called Caomhanach (of St. Kevin's church), which was anglicised to Kavanagh.

Donal did not become a king but did acquire large tracts of land in Wexford and Carlow, which remain strongholds of the Kavanaghs to this day. Borris House in Co. Carlow was once a MacMurrough-Kavanagh house and is being restored. In Co. Wexford, the MacMurrough-Kavanagh connection is maintained in two castles: Ferns Castle was owned and occupied by the family from 1402 to 1550, and the castle in Enniscorthy, now the Wexford county museum, was once also a MacMurrough - Kavanagh stronghold. The Kavanaghs were Kings of Leinster until Henry VIII's reign in the 16th century.