His critically acclaimed novel, At Swim, Two Boys (2001), earned him the highest advance ever paid for an Irish novel and frequent praise as the natural successor to James Joyce, Flann O'Brien and Samuel Beckett.
[1] Terry Pender commented on At Swim, Two Boys: "With only this work O'Neill can take his rightful place among the great Irish writers beginning with Joyce and ending with Roddy Doyle".
[3] O'Neill lists as his favourite books: Ulysses, by James Joyce, The Last of the Wine, by Mary Renault, Hadrian the Seventh, by Frederick Rolfe (Frederick Baron Corvo), The History of the Decline and Fall of the Roman Empire, by Edward Gibbon, The Leopard, by Giuseppe Tomasi di Lampedusa, The Siege of Krishnapur, by J. G. Farrell, One Hundred Years of Solitude, by Gabriel García Márquez, The Third Policeman, by Flann O'Brien, The Swimming Pool Library, by Alan Hollinghurst, and The Lost Language of Cranes by David Leavitt.
His frequent excursions to London, though, are how O'Neill met with and began a relationship with TV presenter Russell Harty, who encouraged him to publish his work.
The British tabloid press, who had run multiple exposés about Harty during his fatal illness, approached O'Neill offering him money to sell his story.
Harty's family had O'Neill, who had no legal standing in the estate, removed from the presenter's house; during this difficult period he eventually became homeless.
O'Neill struggled to write, parted company with both his agent and publisher, and took a job as a night porter at the Cassel Hospital, a psychiatric institution in Richmond, London from 1990 to 2000.