He was educated at the Patrician Monastery, Tullow, County Carlow, and studied Moral and Mental Science at the Royal University, where he graduated in 1907 in First Place with First Class Honours. Subsequently, he studied at King's Inns, Dublin, and was called to the Irish Bar in 1911, where he practised mainly in real property.
He was awarded a Master of Arts degree in 1908. He joined the Land Commission, where he acquired an intimate knowledge of the system of real property and land tenure in Ireland. He stood as a pro-Treaty Sinn Féin candidate at the 1922 general election for the Wexford constituency but was not elected.
In 1922 he was appointed by the Provisional Government of the Irish Free State to the Irish Free State Constitution Commission to draft the Constitution of the Irish Free State, commonly known as the Irish Free State Constitution Commission. It prepared a draft Constitution. He was thus one of the constitutional architects of the Irish Free State.
In 1923 he was appointed to the Judiciary Commission by the Government of the Irish Free State, on a reference from the Government to establish a new system for the administration of justice in accordance with the Constitution of the Irish Free State. The Judiciary Commission was chaired by the last Lord Chief Justice of Ireland (who had also been the last Lord Chancellor of Ireland). It drafted legislation for a new system of courts, including a High Court and a Supreme Court, and provided for the abolition of the Irish Court of Appeal.
He was also a delegate of the Irish Free State to the League of Nations in the same year. On 7 June 1924 he was appointed the second of the Irish Free State when Hugh Kennedy was appointed Chief Justice. His photograph appears at.
On 9 January 1926 he was appointed a Justice of the High Court, upon which he served until he was appointed to the Supreme Court in 1940. He was also Chairman of the Irish Legal Terms Advisory Committee from 14 May 1948 to 13 May 1953. Another High Court judge Kenneth Deale writing extra judicially in " Beyond Any Reasonable Doubt ", a collection of celebrated Irish murder trials, offers some interesting insights into O'Byrne's strengths and weaknesses as a judge.
Deale praises him as a sound and experienced lawyer, conscientious, principled and level-headed. However he believed he had one flaw- he was excessively strong minded and having made up his mind was most reluctant to change it. This in Deale's view was a problem especially in criminal trials where it is hard to believe a jury would not be greatly influenced by the summing up of so formidable and strong minded a judge.
He lived at Stonehurst, Killiney, County Dublin. He died in his office on 14 January 1954. His widow brought a celebrated test case arguing that judges could not be required to pay income tax as this breached the Constitutional guarantee that their incomes shall not be reduced.
The Supreme Court decided by a 3-2 majority that judges are liable to pay income tax. One of Mr. Justice O'Byrne's sons was Paddy O'Byrne who became one of the best known radio presenters in South Africa.
He was appointed King's Counsel in 1924, becoming the last member of counsel in the Free State to be appointed (thereafter, counsel were appointed as Senior Counsel).