The name "Dennehy" is spelled with two syllables: /dɛn.i/. The first syllable is pronounced with an open "e" sound, like in the word "den" or "pen." The second syllable is pronounced with a short "i" sound, like in the word "it" or "sit." This spelling is common for Irish surnames, and it is thought to have originated from the Irish word "dún na nEach," which means "the fort of the horses." The name is most famously associated with actor Brian Dennehy.
The word "Dennehy" is derived from the Irish surname Ó Duinnshléibhe, which translates to "descendant of Duinnshléibhe". The surname's origin comes from combining the Gaelic words "dubh" (meaning "black") and "sliabh" (meaning "mountain"). Therefore, Ó Duinnshléibhe signifies "descendant of the one from the black mountain". Over time, Ó Duinnshléibhe evolved into "Dennehy" through anglicization.