In this position, he was hugely influential at the Committee on the Operation of Dominion Legislation and at the Imperial Conference in 1930 (jointly with representatives of Australia, Canada, New Zealand, South Africa and the United Kingdom).

The Statute of Westminster that emerged from these meetings gave greater power to dominions in the Commonwealth like the Irish Free State.

The Irish Republican Army began to disrupt Cumann na nGaedhael public meetings, and in turn, a pro-Cumann na nGaedhael paramilitary called the Army Comrades Association (later better known as the Blueshirts) was created to counteract the IRA and disrupt Fianna Fáil meetings.

As the links between the Blueshirts and Cumann na nGaedhael rapidly developed, sitting CnaG Teachta Dála Thomas F. O'Higgins became the leader of the ACA.

When the National University Dáil constituency was abolished in 1937 (before being recreated in the Seanad in 1938), McGilligan was elected as TD for Dublin North-West.

A later Attorney General, John M. Kelly in the preface to his definitive text, The Irish Constitution (1980), noted the remarkable number of senior judges who were former students of McGilligan and suggested that given his own firm belief in the value of judicial review, he deserves much of the credit for the remarkable development of Irish law in this field since the early 1960s.