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 the Irish Bar, establishing a successful legal practice and participating in significant constitutional cases, such as the Land Annuities controversy, 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.