[1] The hallmark of his style was his carefully observed realism and his highly finished surfaces, the result of a virtuoso painting technique.

He attended Dundee School of Art where he met his lifelong companion, the painter Henry (Harry) Robertson Craig.

[7] In the autumn of that year he enrolled at the Dundee School of Art, for a four-year Diploma course in Drawing and Painting under James McIntosh Patrick RSA and Edward Baird.

[6][9] Hennessy played a full part in the social activities of the college, winning a fancy dress award at the Christmas revels in 1935[10] and producing a ballet "Paradise Lost" the following year.

[17] On his return to Scotland he was selected for the residential summer school course at Hospitalfield House near Arbroath under James Cowie.

[1][21] James Mackintosh Patrick when asked fifty years later if he remembered his pupil Hennessy, replied, I can recall him only because he was outstanding.

[22] On arrival in Dublin, the twenty four year old Hennessy was offered an exhibition in December 1939 at the Country Shop on St Stephens Green which was opened by Mainie Jellett.

In 1940 he was invited to join the Society of Dublin Painters and held regular annual exhibitions of his work there during the forties and early fifties.

During the late 1940s, Hennessy undertook a portrait of Francis Bacon, which remains unfinished, but which also serves to link Hennessy to the artistic social network connected with Bacon, including Lucian Freud, John Craxton, Peter Watson, Edward James, Salvador Dalí, and Cyril Connolly.

[1] In 1947, Time magazine selected Hennessy as one of Irelands outstanding painters, in recognition of the important position he had then attained in the art world.

[1] Also in 1951 Hennessy visited Italy taking in Venice and Sicily and returning to Dublin with many of his canvases painted abroad.

Hennessy spent a lot of his summer months during this period on trips abroad to France, Italy, Greece and Spain.

Hennessy's exhibitions at the Ritchie Hendriks gallery had for many years enjoyed favourable reviews from the art critics but in the 1960s this changed.

Some critics claimed his paintings failed to communicate any genuine "personal" vision and criticised his use of ugly colour.

[29] One critic wrote that Hennessy deserves admiration of a sort for ploughing such a lonely furrow and that he is one of the true outsiders of latter day Irish painting.

[30] The North American market was extremely lucrative for Hennessy and by the end of the decade he was selling more of his work in the US than in Ireland.

In 1968 Hennessy finally moved to Tangier, Morocco on a permanent basis and in 1970 sold his studio on Raglan Lane Dublin to his doctor.