The surname Kavanagh or Cavanaugh and the other variants of the name are derived from the Irish Gaelic name Caomhánach, which means 'a student or follower of St. Caomhan' and was first used by Domhnall, eldest son of the 12th century King of Leinster Diarmait mac Murchada (Dermot MacMurrough).

Domhnall was fostered for his training and education at the monastery of St. Caomhan at which is now known as Kilcavan in the Barony of Gorey, County Wexford. He became known as Caomhánach, distinguishing him from the other sons of Diarmait. His brother Eanna became known as Eanna Ceinnsealach, the name of the Clan land holdings and became the progenitor of the Kinsella Clan.

The Norman knight "Strongbow", Earl of Stirgoil married Domhnall's sister Aoife and attempted to commandeer the Kingship of Leinster following the death of King Dermot MacMurrough in 1171 A.D. However under Irish (Brehon) law, Strongbow had absolutely no basis for his claim and was promptly rejected by the Irish chiefs who elected Domhnall Caomhánach as King.

Strongbow viciously executed Domhnall's son whom he held as hostage and then bribed two Irish warriors in his own army to kill Domhnall during the Battle of Naas in 1175 A.D. These treacherous acts characterized the relationship between the Normans and the Irish for the following centuries.