In 1913 he established the Irish Volunteers and served as their Chief-of-Staff. He held this position at the outbreak of the Easter Rising but had no role in it or its planning, which was carried out by IRB infiltrators. MacNeill helped countermand the Easter Monday uprising, after learning about it and confronting Patrick Pearse, by placing a last-minute news advertisement advising Volunteers not to take part. He was later elected to the First Dáil as a member of Sinn Féin.
MacNeill was born John McNeill, one of five children born to Archibald McNeill, a Roman Catholic working class "baker, sailor and merchant", and his wife, Rosetta (née McAuley) McNeill, also a Catholic. He was reared in Glenarm, County Antrim, an area which "still retained some Irish-language traditions."
He was educated at St Malachy's College (Belfast) and Queen's College, Belfast. MacNeill had an enormous interest in Irish history and immersed himself in its study. In 1888 he achieved a BA degree in economics, jurisprudence and constitutional history and then worked as civil-servant clerk.
In 1893 he co-founded the Gaelic League, along with Douglas Hyde; he was unpaid secretary from 1893 to 1897, and then became the initial editor of the League’s official newspaper An Claidheamh Soluis (1899–1901). He was also editor of the Gaelic Journal from 1894 to 1899. In 1908 he was appointed professor of early Irish history at University College Dublin (UCD).
He married Agnes Moore on 19 April 1898; the couple had eight children, four sons and four daughters.
The Gaelic League was from the start strictly non-political, but in 1915 a proposal was put forward to abandon this policy and become a semi-political organisation. MacNeill strongly supported this, and rallied to his side a majority of delegates at the 1915 Oireachtas. Douglas Hyde, a non-political Protestant, who had co-founded the League and been its President for 22 years, resigned immediately afterwards.
Through the Gaelic League, MacNeill met members of Sinn Féin, the Irish Republican Brotherhood, and other nationalists and republicans. One such colleague, The O'Rahilly, ran the league's newspaper, An Claidheamh Soluis, and in October 1913 asked MacNeill to write an editorial for it on a subject more broad than Irish language issues. MacNeill submitted a piece called "The North Began", encouraging formation of a nationalist volunteer force committed to Home Rule, much as the Unionists had done earlier that year with the Ulster Volunteers to thwart Home Rule.
Bulmer Hobson, a member of the IRB, approached MacNeill about bringing this idea to fruition, and, through a series of meetings, MacNeill became chairman of the council that formed the Irish Volunteers, later becoming its chief of staff. 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.
The Irish Volunteers had been infiltrated by the Irish Republican Brotherhood, which planned on using the organisation to stage an armed rebellion, with the goal of separating Ireland from the United Kingdom and establishing a republic. The entry of the UK into the First World War was, in their view, a perfect opportunity to do so. With the co-operation of James Connolly and the Irish Citizen Army, a secret council of IRB officials planned a general rising at Easter, 1916. On the Wednesday before, they presented MacNeill with a letter, allegedly stolen from high-ranking British staff in Dublin Castle, indicating that the British were going to arrest him and all the other nationalist leaders. Unbeknownst to MacNeill, the letter—called the Castle Document—was a forgery.
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 mobilisation of the Irish Volunteers would be a defensive act. However, after learning of Casement's arrest and the loss of the promised German arms, and confronting Patrick Pearse, MacNeill countermanded the order for the Rising in print, greatly reducing the number of volunteers who reported for duty on the day of the Easter Rising.
Pearse, Connolly, and the others agreed that the uprising would go ahead anyway, but it began one day later than originally intended to ensure the authorities were taken by surprise. Beginning on Easter Monday, 24 April 1916, the rising lasted less than a week. After the surrender of the rebels, MacNeill was arrested, although he had taken no part in the insurrection.
MacNeill was released from prison in 1917 and was elected Member of Parliament for the National University of Ireland and Londonderry City constituencies for Sinn Féin in the 1918 general election. In line with abstentionist Sinn Féin policy, he refused to take his seat in the British House of Commons and sat instead in the newly convened Dáil Éireann. He was a member of the Parliament of Northern Ireland for Londonderry during 1921–25 although he never took his seat. In 1921 he supported the Anglo-Irish Treaty. In 1922 he was in a minority of pro-Treaty delegates at the Irish Race Convention in Paris. Following the establishment of the Irish Free State, he became Minister for Education in its first government.
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.
MacNeill's family was split on the Treaty issue. 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. Two other sons, Niall and Turloch, served as officers in the Free State Army. One of Eoin's brothers, James McNeill, was the second and penultimate Governor-General of the Irish Free State.
In 1924 the Irish Boundary Commission was set up to renegotiate the border between Northern Ireland and the Irish Free State; MacNeill represented the Free State. On 7 November 1925, a conservative British newspaper, The Morning Post, published a leaked map showing a part of eastern County Donegal (mainly The Laggan district) that was to be transferred to Northern Ireland; the opposite of the main aims of the Commission. Perhaps embarrassed by this and more so because, he said, that it had declined to respect the terms of the Treaty, McNeill resigned from the Commission on 20 November. He resigned on 24 November as Minister for Education, a position unrelated to his work on the Commission.
On 3 December 1925 the Free State government agreed with the governments in London and Belfast to end its onerous Treaty requirement to pay its share of the United Kingdom's "imperial debt", and in exchange it agreed that the 1920 boundary would remain as it was, over-riding the Commission. This angered many nationalists and MacNeill was the subject of much criticism, though in reality he and the commission had been sidestepped by the inter-governmental debt renegotiation. In any case, despite his resignations, the intergovernmental boundary deal was approved by a Dáil vote of 71–20 on 10 December 1925, and MacNeill is listed as voting with the majority in favour. He lost his Dáil seat at the June 1927 election.
MacNeill was an important scholar of Irish history, and among the first to study Early Irish law, offering both his own interpretations, which at times were coloured by his nationalism, and offering translations into English. He was also the first to uncover the nature of succession in Irish kingship and his theories are the foundation for modern ideas on the subject. He was a contributor to the RIA's Clare Island Survey, recording the Irish place names of the island. On 25 February 1911 he delivered the inaugural address on "Academic Education and Practical Politics" to the Legal and Economic Society of UCD.
He was President of the Royal Society of Antiquaries of Ireland from 1937 to 1940 and President of the Royal Irish Academy from 1940 to 1943.
He retired from politics completely and became Chairman of the Irish Manuscripts Commission. In his later years he devoted his life to scholarship, he published a number of books on Irish history. MacNeill died in Dublin of natural causes, aged 78.
One of his grandsons is the former Tánaiste and Minister for Justice, Equality and Law Reform, Michael McDowell. Another grandson, Myles Tierney, served as a member of Dublin County Council, where he was Fine Gael whip on the Council.