Gavan Duffy returned to the Irish Bar and built up a large practice and was engaged in some notable constitutional cases such as the Land Annuities controversy in which he claimed that the Irish Free State could not be bound either in honour or in law to hand over annuities to Britain. He was appointed Senior Counsel in 1930 and Judge of the High Court in 1936.
Gavan Duffy returned to practicing law in Ireland, building a prominent legal career and engaging in significant constitutional cases, including the Land Annuities dispute where he argued that the Irish Free State was not legally obligated to transfer annuities to Britain. He was appointed Senior Counsel in 1930 and Judge of the High Court in 1936.