On learning of the plans to launch an uprising on Easter Sunday, and after confronting Pearse about it, MacNeill issued a countermanding order, placing a last-minute newspaper advertisement instructing Volunteers not to participate.

He achieved a BA degree in economics, jurisprudence and constitutional history in 1888, and then worked in the British Civil Service.

[8] The Gaelic League was from the start strictly non-political, but in 1915, a proposal was put forward to abandon that policy and become a semi-political organisation.

One such colleague, The O'Rahilly, ran the league's newspaper An Claidheamh Soluis, and in October 1913 they asked MacNeill to write an editorial for it on a subject broader than Irish language issues.

Unlike the IRB, MacNeill was opposed to the idea of an armed rebellion, except in resisting any suppression of the Volunteers, seeing little hope of success in open battle against the British army.

With the cooperation of James Connolly and the Irish Citizen Army, a secret council of IRB officials planned a general rising at Easter 1916.

[10] When MacNeill learned about the IRB's plans, and when he was informed that Roger Casement was about to land in County Kerry with a shipment of German arms, he was reluctantly persuaded to go along with them, believing British action was now imminent and that mobilization of the Irish Volunteers would be justified as a defensive act.

However, after learning that the German arms shipment had been intercepted and Casement arrested, and having confronted Patrick Pearse, who refused to relent, MacNeill countermanded the order for the Rising by sending written messages to leaders around the country, and placing a notice in the Sunday Independent cancelling the planned "manoeuvres".

[12] Pearse, Connolly and the others agreed that the uprising would go ahead anyway, but it began one day later than originally intended to ensure that the authorities were taken by surprise.

[17] He strongly supported the execution of Richard Barrett, Liam Mellows, Joe McKelvey and Rory O'Connor during the Irish Civil War.

[18] In 1923, MacNeill, a committed internationalist, was also a key member of the diplomatic team that oversaw Ireland's entry to the League of Nations.

One son, Brian, took the anti-Treaty side and was killed in disputed circumstances near Sligo by Free State troops during the Irish Civil War in September 1922.

[20] Two other sons, Niall and Turloch, as well as nephew Hugo MacNeill, served as officers in the Free State Army.

[29] On 25 February 1911, he delivered the inaugural address on "Academic Education and Practical Politics" to the Legal and Economic Society of University College Dublin.

[33] His grandson Michael McDowell served as Tánaiste, Minister for Justice, Equality and Law Reform, TD and a Senator.