The surname Kavanagh or Cavanaugh and the other variants of the name are derived from the adjectival Irish Gaelic name Caomhánach. This was the name applied to Domhnall, eldest son of the 12th century King of Leinster Diarmait mac Murchada (Dermot MacMurrough).
Domhnall was fostered, according to Irish custom, by the family attached to the monastery of St. Caomhan at Kilcavan in the Barony of Gorey, County Wexford. He thus became known as Caomnach. His brother Eanna became known as Eanna Ceinnsealach, also an adjectival name derived from the Clan and its territory. Eanna became the progenitor of the Kinsella Clan.