He represented Fianna Fáil politician Des Hanafin in a case seeking to have Fifteenth Amendment of the Constitution of Ireland, permitting divorce, declared to be unconstitutional.
[3] Early in his career on the bench in High Court, he clashed with the government over the treatment of vulnerable young people.
[12] In 2000 he put in place a mandatory injunction requiring government ministers to provide specialist support care for underage patient.
[10] Kelly said that while presiding over the Commercial Court he witnessed "national and international fraud, sharp practice, chicanery and dishonesty".
[3] Upon his appointment, the Director General of the Law Society of Ireland described him as a "fearlessly independent judge with a ferocious work ethic" and said he had a "first-class legal mind".
[22] The Irish Times reported that he was one of three judges considered for the role of Chief Justice of Ireland upon the retirement of Susan Denham in 2017, though only Frank Clarke's name was put forward for selection by the cabinet.
[23] As President, he introduced greater safeguards for the ward of court process, re-introducing independent medical visitors.
Mícheál O’Higgins, then President of the Bar Council, praised what he considered Kelly's "competence, rigour, propriety and independence".
[25] Kelly is an adjunct professor of law at Maynooth University, serving on the Council of the Royal College of Surgeons in Ireland.