In 1600, at the age of 15, Cahir joined the forces of the English Governor of Derry, Sir Henry Docwra, who were fighting to defeat Tyrone's Rebellion.

By this time, the sympathetic Docwra had been replaced by Sir George Paulet,[5] who took a much harder view of the Gaelic lords, even loyalists like O'Doherty.

Paulet, governor of Derry, overreacted to reports that O'Doherty and a number of his followers were assembling for a planned revolt and marched out with troops.

O'Doherty was in fact taking part in a wood-cutting expedition to the Canmoyre Woods near his home, rather than assembling a rebellion as Paulet had been led to believe.

Instead, Lord Deputy Chichester ordered that O'Doherty put up a huge surety bond of more than £1,000, its repayment conditional on his future behaviour, and banned him from leaving Ireland – forbidding him to go to London to lobby King James.

In January 1608, Sir Cahir, as he was now, sat on the Irish jury that confirmed the Act of Attainder against the absent Earl of Tyrconnell, stripping him of his lands and title for treason.

He was encouraged by one of his neighbours, Sir Niall Garve O'Donnell, who was possibly duplicitous and hoped to be awarded O'Doherty's lands in the event of a failed rebellion.

[10] Even at this point, O'Doherty remained reluctant to rebel and with "tears in his eyes" he approached English officers asking them how he could regain the Lord Deputy's favour.

However, Chichester later admitted his part of the responsibility for his bad treatment of O'Doherty after the Canmoyre Woods incident, observing that "all men believed that he had been wronged".

[18] The English military in Ireland had few troops ready to respond to the outbreak of violence, but Chichester mustered the male inhabitants of Dublin and surroundings and sent what forces he could northwards immediately.

It still remained a distinct prospect that the revolt would lead to the Crown offering a favourable settlement to O'Doherty rather than face an expensive war, a common occurrence in Ireland during previous centuries.

But some of his gathering supporters believed the Earl of Tyrone was about to return with Spanish help, possibly as a first stage to restoring Catholicism in England and Scotland as well as Ireland.

Chichester offered to step down from his post once the rebellion was put down, and King James was said to have been "persuaded that the mistaken conduct of the present Viceroy has much to do with these events".

[20] In the meantime Chichester despatched 700 troops under Sir Richard Wingfield, encouraging them to launch a "thick and short" campaign in Ulster.

Wingfield's forces overran O'Doherty's own territory in Inishowen, capturing the town of Buncrana and recovering the ruined remains of Derry.

[22] The loss of his own castle badly damaged the morale of O'Doherty's supporters, who pressured him into seeking a direct confrontation with Wingfield's army and threatened to leave him if he did not.

[23] Gathering his forces, Sir Cahir marched with 1,000 men and met the Crown's troops in July 1608 at Kilmacrennan near Letterkenny in County Donegal, on ground selected by O'Doherty that made it difficult for the enemy's cavalry to deploy.

[26] To receive "Pelham's Pardon", the commander of the castle, Sir Mulmory McSweeney, began to kill his fellow defenders to present their heads to the besiegers.

The Plantation was intended to break up the traditional Gaelic model of a handful of great lords such as Tyrone, replacing them with smaller landowners whose primary loyalty was to the Crown.