O'Shaughnessy was born on December 22, 1850 in Dublin, son of Thomas O'Shaughnessy and Mary Lopdell. He married Catherine Newman in 1879 and they had four children. He died at his home in Fitzwilliam Square, Dublin on March 7, 1933. Educated at Queens College Galway, he was called to the Irish Bar in 1874 and to the English Bar by Middle Temple in 1894.
O'Shaughnessy carried out his practice on the Connaught and North Eastern Circuit. He served as counsel to the Plaintiffs in relation to the disastrous rail accident during a school outing from Armagh to Newry. O'Shaughnessy won a great reputation from this trial, and took silk (an informal term for Queen's Counsel) soon after. For fifteen years, he was one of the most influential, effective and well-paid barristers of the Dublin Four Courts, and was regarded as a mentor to younger barristers. Maurice Healy remarked that there was a rather unfriendly rivalry between himself and William Huston Dodd as to which of them was "Leader of the Bar".  He was sworn in as Recorder of Dublin in 1905. He would be the last to hold this position, a role which stretched back to James Stanihurst in 1564. The Recordership was abolished in 1924 and O'Shaughnessy became a judge in the High Court of the Irish Free State. He resigned in 1925 and received a knighthood in 1927. He was a Bencher of the King's Inn and was sworn in as a member of the Irish Privy Council in 1912. He was a member of the Reform Club.
- Healy, Maurice The Old Munster Circuit 1939 Mercier Press edition p.41
- Healy p.41
- Healy p.239