He was born to Geraldine and Harry (both international tennis players), while his grandfather Henry Barniville was a member of Seanad Éireann and surgeon.

[11] He appeared regularly in the European Court of Justice representing Ireland.

[7] In 2016, the Minister for Justice and Equality Frances Fitzgerald sought clarifications from the Bar Council on the procedure of Barniville's appointment to the Legal Services Regulatory Authority.

[4] He has acted as a governor of the National Maternity Hospital, Dublin and was the chair of Irish Rule of Law International.

[21] He was previously the Judge in Charge of the Strategic Infrastructure List,[7] hearing appeals of An Bord Pleanála decisions, including for developments at Saint Anne's Park and Cherrywood, Dublin.

[24] The appointment of Maurice Collins as a part-time commissioner of the Law Reform Commission permitted a vacancy on the Court of Appeal.

[25][26] Barniville was nominated to become President of the High Court in June 2022, to succeed Mary Irvine on her retirement in July 2022.