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.
Another High Court judge, Kenneth Deale writing extrajudicially in "Beyond Any Reasonable Doubt?
",[2] a collection of essays on celebrated Irish murder trials, offers some interesting insights into O'Byrne's strengths and weaknesses as a judge.
In particular, Deale strongly criticised his conduct of the trial of Thomas Kelly, tried in 1936 for the murder of Patrick Henry.
The Supreme Court decided by a 3–2 majority that notwithstanding the guarantee, judges are liable to pay income tax.