[2] His sister Louise Gavan Duffy came to Ireland in 1907, taught in Patrick Pearse's St Ita's school for girls (Scoil Íde), was along with Mary Colum first Secretary of Cumann na mBan, and was out in the Easter Rising of 1916 in the GPO and Jacob's garrisons.

[5] Gavan Duffy published articles and pamphlets urging recognition of Ireland as a sovereign nation at the Paris Peace Conference, which caused increasing embarrassment to the French establishment, who believed his publications were damaging Anglo-French relations.

A final letter of June 1919 demanding recognition and addressed to the French Prime Minister Georges Clemenceau, the chairman of the Peace Conference, was not replied to.

[6] Finally, after publishing a letter he had sent to Clemenceau in protest against the former mistreatment of Terence MacSwiney in prison in 1917, Gavan Duffy was banished from Paris.

When Éamon de Valera chose his plenipotentiaries to negotiate the Anglo-Irish Treaty in 1921, Gavan Duffy was chosen due mainly to his legal expertise.

Duffy spoke of the pressure placed on the Irish delegation to sign the treaty: "...the alternative to our signing that particular Treaty was immediate war...we had to make the choice within three hours and to make it without reference to our Cabinet, to our Parliament or to our people...We lost the Republic in order to save the people of Ireland.

"[7] During the debates which followed in Dáil Éireann, Gavan Duffy stated that he would recommend the Treaty reluctantly but sincerely as he saw no alternative for the desired aim of independence.

[8]Gavan Duffy placed the onus on the people who were responsible for drafting the Constitution of the Irish Free State to frame it in accordance with the terms of the Treaty.

He disagreed, however, with Griffith's decision to show the draft constitution to British Prime Minister Lloyd George, who immediately ordered that references to the King had to be inserted as well as an Oath of Allegiance.

If that be so, we must be asked to believe that every one of those who have gone before us in previous fights, and who in the end have had to lay down their arms or surrender to avert a greater evil to the people, have likewise been guilty of a breach of principle.

He acted as an unofficial legal advisor to Éamon de Valera during the drafting of the 1937 Constitution of Ireland and was consulted on many issues pertaining to it.

In 1946, at the height of his legal career, he was appointed President of the High Court, a position he held for the rest of his life.

His judgement applied the ne temere decree to the letter, as de Valera's 1937 Irish Constitution gave the Roman Catholic Church in Ireland a "special position".