He attended Trinity College, Dublin (TCD), to study for an MPhil in medieval history, completing his masters in 1996.
His book, War, politics and the Irish of Leinster, 1156–1606, was published by Four Courts Press in 2003, receiving critical acclaim in a number of international journals such as Speculum (2005),[2] The English Historical Review (2005)[3] and Celtica (2007).
[4] In September 2003, he was awarded a post doctoral fellowship at the Mícheál Ó Cléirigh Institute for the Study of Irish History and Civilisation at University College, Dublin (UCD), holding that position until its expiration in late 2006.
In the interim years, O'Byrne, with Dr. Edel Breathnach, organised an international conference in 2005 at UCD entitled "The March in Medieval West, 1000–1500", attracting several renowned historians.
[citation needed] With Dr. Jenifer Ní Ghrádaigh of University College, Cork (UCC), he has edited the proceedings of this conference published in 2010.