Born in Co Antrim to middle-class Catholic parents, he was educated at St Malachy’s College, Belfast and became law clerk. In 1893, together with Douglas Hyde and others he founded the Gaelic League, an organisation devoted to the preservation of the Irish language, literature, and traditional culture. A brilliant historian and linguist, he was the first professor of early and medieval Irish history at University College Dublin.
In an article entitled ‘The North Began’ in the paper An Claidheamh Soluis (November 1913), McNeill advocated the formation of a national volunteer force on the lines of the Ulster Volunteer Force. MacNeill became Chief of Staff of the Irish Volunteers — its main aim to safeguard Home Rule and their numbers swelled to 170,000 before the split in September 1914 and the defection of the vast majority of members to Redmond. The Irish Republican Brotherhood, which wanted full independence, would recruit the remaining Irish Volunteers for the Rising.
In 1916 MacNeill countermanded the orders of the IRB Military Council for manoeuvres on Easter Sunday when he found out that they were a cover for the Rising — with which he disagreed. Argument persists as to the effects of MacNeill’s countermand on the Easter Rising, and its outcome. He was sentenced to penal servitude for life in 1916 for his connections to the insurrection but was released one year later. MacNeill later supported the Treaty and became Minister for Education in 1922, and was a member of the ill-fated Boundary Commission (1925).