Niall Garbh O'Donnell

Sir Niall Garbh O'Donnell (Irish: Niall Garbh Ó Domhnaill; c. 1569 – 1626) was an Irish nobleman and soldier who alternately rebelled against and supported English rule in Ireland. During the Nine Years' War he defected from the Irish confederacy and sided with the Crown against his cousin Hugh Roe O'Donnell, with the aim of restoring the lordship of Tyrconnell to his own branch of the O'Donnell clan. He was inaugurated as the O'Donnell clan chief in April 1603. He turned against the Crown in 1608, following years of land rights disputes.

Although Niall's paternal grandfather Calvagh ruled Tyrconnell, in 1557 the lordship was seized by Niall's half-uncle Hugh McManus O'Donnell. Following a violent succession conflict, Hugh McManus's son Hugh Roe—Niall's younger cousin—succeeded to the lordship in 1592. A long lasting bitterness ensued between the two cousins.

Niall initially fought for Hugh Roe and the Irish confederacy, but in 1600 he entered secret negotiations with English commander Henry Docwra. The government hoped to harness Niall's feud against Hugh Roe and promised him the lordship of Tyrconnell for his military assistance. Niall defected in October bringing many followers with him. Niall's skills in guerrilla warfare emboldened the royal troops and allowed Docwra to significantly weaken Irish forces in Ulster. However, Niall's desire to rule Tyrconnell as a sovereign entity was incompatible with the Plantation of Ulster, and after the war ended, the majority of Tyrconnell's land was granted back to Hugh Roe's immediate family.

In early 1608 Niall covertly instigated fellow spurned loyalist Cahir O'Doherty to launch a rebellion in Derry, but he was quickly implicated and put on trial for treason. Faced with a sympathetic jury that would almost certainly acquit, the government sent Niall to the Tower of London in 1609, where he remained until his death seventeen years later.