O'Neill left Ireland at a young age and spent most of his life as a mercenary in the Spanish Army serving against the Dutch in Flanders during the Eighty Years' War.

O'Neill's later years were marked by infighting amongst the Confederates, and in 1647 he led his army to seize power in the capital of Kilkenny.

He initially rejected a treaty of alliance between the Confederates and the Irish Royalists, but faced with the Cromwellian invasion he changed his mind.

Shortly after agreeing to an alliance with the 1st Marquess of Ormond, the Lord Lieutenant of Ireland, in which he was promised an Earldom, O'Neill died on Tuesday, 6 November 1649.

Another of his nephews, Hugh Dubh O'Neill fought in the Confederate Wars and famously inflicted heavy casualties on Oliver Cromwell's New Model Army during the Siege of Clonmel in 1650.

He distinguished himself notably at the Siege of Arras in 1640, where he commanded the Spanish garrison and held out for 48 days with 2,000 men (many of whom were fellow Irishmen), against a French army of 35,000.

Despite a failed attempt to seize Dublin Castle, the rebels enjoyed success across Ulster and the uprising spread to other parts of the country.

Sir Phelim O'Neill resigned the northern command of the Irish rebellion in Eoghan Rua's favour and escorted him from Lough Swilly to Charlemont.

[8] But distrust between the kinsmen was complicated by differences between Owen Roe O'Neill and the Catholic Confederation which met at Kilkenny in October 1642.

Mainly because Preston had been given the available military resources, O'Neill was outnumbered by the Scottish Covenanter army that had landed in Ulster in 1642.

Following a defeat at the Battle of Clones, O'Neill had to abandon central Ulster and was followed by thousands of refugees, fleeing the revenge of the Scottish soldiers who inflicted terrible attacks on Irish civilians, persuaded by Protestant propaganda alleging atrocities against Anglo-Scottish settlers in the rebellion of 1641.

This poorly supplied force nevertheless gained a bad reputation for plundering and robbing friendly civilians around its quarters in northern Leinster and southern Ulster.

After the midlands campaign, O'Neill was ordered to return to Ulster with Lord Castlehaven to conduct offensive operations against the territory held by the Covenanters.

At one point, O'Neill even considered leaving and returning to Spanish service, but was ultimately compelled not to with the arrival of weapons and reinforcements sent by the Papacy.

On 5 June 1646, despite being outnumbered and outgunned, O'Neill utterly routed Monro at the Battle of Benburb,[5] on the Blackwater killing or capturing 3,000 Scots.

After the defeat in the conflict, he had fled Ireland during the Flight of the Earls in an attempt to gain support from Catholic powers in Continental Europe, where he died in Rome in 1616.

This campaign had mixed success, but O'Neill was recalled to Leinster after Preston was defeated by the Parliamentarians near Trim, County Meath.

The Irish Confederates suffered heavy military defeats the following year at the hands of Parliamentarian forces in Ireland at Dungans Hill and Knocknanauss, leading to a moderation of their demands and a new peace deal with the Royalists.

This time O'Neill was alone among the Irish generals in rejecting the peace deal and found himself isolated by the departure of the Papal Nuncio from Ireland on 23 February 1649.

He made overtures for alliance to Monck, who was in command of the Parliamentarians in the north, to obtain supplies for his forces, and at one stage even tried to make a separate treaty with the English Parliament against the Royalists in Ireland.

O'Neill later relieved Parliamentarian commander Charles Coote, who was under siege in Derry by the Covenanters, in return for a herd of 2,000 cattle.

However, upon hearing of Oliver Cromwell's landing in Ireland and the subsequent Sack of Drogheda, and failing to obtain any better terms from the Parliamentarian forces, he turned once more to Ormond and the Catholic confederates, with whom he prepared to co-operate more earnestly when Cromwell's arrival in Ireland in August 1649 brought a war of horror.

[11] O'Neill died on 6 November 1649[21] at the O'Reilly stronghold of Cloughoughter Castle on an island in Lough Oughter in County Cavan.

Thomas Davis wrote a song about O'Neill, "The Lament for Owen Roe", first published in the Young Ireland newspaper The Nation.

Drawing on an older melody composed by the harp player Turlough Carolan, it portrays his death as an assassination and the main cause of the subsequent defeat to Cromwell's English Republican forces.

O'Neill is commemorated in the names of several Gaelic Athletic Association clubs in Northern Ireland, including (in Armagh) Eoghan Ruadh Middletown; (in Derry) CLG Eoghan Rua, Coleraine; (in Dublin) St Oliver Plunketts/Eoghan Ruadh GAA, and (in Tyrone) Brackaville Owen Roes GFC; Owen Roe O'Neill's GAC, Leckpatrick; Eoghan Ruadh, Dungannon GAA, in Down, Kilcoo Owen Roes GAC and the defunct Benburb Eoghan Ruadh GAC The Irish Army opened a new barracks in 1990, to replace the old military post in Cavan Town since 1707?, and named it "Dún Uí Néill" (O'Neill's Fort or Fort of O'Neill).