[2] In some works, he used jazz elements and took inspiration from funfairs and industrial noise, anticipating trends associated with the group of "Les Six".
Ten years of travel followed, including France, Belgium, Switzerland, and Ireland, with a home base in Italy, before he settled in Paris around 1903.
Although not from Brittany, Hennessy was a member of the short-lived Association des Compositeurs Breton before World War I and continued to mix with their members including Paul Le Flem, Paul Ladmirault, Maurice Duhamel, Louis Aubert, Louis Vuillemin, Lucien Haudebert and others throughout the 1920s.
[10] Hennessy died in 1929 from an embolic following a routine operation;[11] the composer Georges Migot gave the funeral speech.
This changed only after his membership in the Association des Compositeurs Bretons from 1912 and the integration of elements from the traditional music of Ireland, Scotland and Brittany into his compositions.
Many of Hennessy's pieces with titles ending on terms like "celtique", "gaélique" or "irlandais" are inspired by traditional folk melody and rhythms, but he rarely quotes actual folktunes.
In a French obituary, he was called "le barde de l'Irlande" and is credited as having saved "l'ancienne mélodie celtique".
[14] Hennessy was very critical of the contemporary avant-garde, particularly of Arnold Schoenberg, and wrote numerous sarcastic and pessimistic letters and comments in the musical press.