In May 1903, McDonnell had opposed a Union motion moved by John Maynard Keynes: "that Home Rule for Ireland is beyond the sphere of practical politics".
During his time in West Africa McDonnell served as Assistant District Commissioner in the Gold Coast, magistrate in The Gambia, and Attorney-General and Acting Chief Justice in Sierra Leone.
[4] McDonnell was forced into early retirement – historian Matthew Hughes speaks of his being dismissed from office[5] – in October 1936.
[6][7][8] After retiring from the bench and returning to London, McDonnell took up advocacy on behalf of the Arab cause in Palestine: he published a number of articles in which he attacked Britain's pro-Zionist policy in Palestine and in 1939, he was an adviser to the Arab delegation concerning the 1915–1916 correspondence between Sir Henry McMahon and the Sharif Hussayn of Mecca.
The delegation disputed the government's claim that McMahon's letters excluded the territory and people of Palestine from the British promise that Arab self-determination.