[3] His mother, Eva Daisy (1897-1991), was the daughter of Henry Randolph Randell, a house painter and decorator from an old Sussex family of Saxon origin.

His first book was published in 1968: Wales: a Nation Again, on the Welsh struggle for political independence, with a foreword by Gwynfor Evans, Plaid Cymru's first MP.

He used his academic background to produce many titles in the field of Celtic Studies and he has written academic articles and papers in the field for journals ranging from The Linguist (London) to The Irish Sword: Journal of the Irish Military History Society (University College Dublin).

He introduced and edited James Connolly: Selected Writing (Penguin, 1973) and wrote a regular column for the Association's newspaper Irish Democrat from 1987 to 2007.

"[8] Apart from his Celtic Studies interests, Ellis has written full-length biographies of the writers H. Rider Haggard, W. E. Johns, Talbot Mundy and E. C. Vivian, as well as critical essays on other popular fiction authors.

[9] As of June 2015 he had published 106 books, 100 short stories, several pamphlets, and numerous academic papers and signed journalistic articles.

[13][14] A book, The Sister Fidelma Mysteries: Essays on the Historical Novels of Peter Tremayne, was published by MacFarland in 2012.

He received an Irish Post Award (1989) for his work on Celtic history, and the French Prix Historia (2010) for best historical crime novel of 2010.