The name Dennehy is principally associate with Irish counties Cork and Kerry. Its original form in Irish is Ó Duineachdha meaning descendant of Duineachaidh. The name goes back to the 11th century recorded in the Battle of Dublin. There are two meanings of the name in the books. One is "humane" and the other is "man from the fairy hills". Humane is more commonly used. This sept is from the same ancestral line as the O Sullivans. Maonach Ui(O)Duineachdha was the founder of the sept.