He spent time in New York in 1962 where he visited the Art Students League of New York and also worked in France, Germany, Greece and the Middle East.
The Pursuit of Diarmuid and Gráinne | Seán Óg Ó Ceallacháin | Diarmuid Gavin | Diarmuid | Diarmuid Whelan | Diarmuid O'Leary | Diarmuid Marsden | Diarmuid Lynch | Diarmuid Connolly |
O'Neill was born and raised in London, the youngest son of Irish parents, Eoghan and Theresa "Terry" O'Neill who were originally from County Kildare and County Dublin.
Hennessy's own musical compositions have been recorded by a wide range of artists such as The Furies, Foster and Allen, Daniel O'Donnell, Diarmuid O'Leary and the Bards and Max Boyce.