Niall initially fought for Hugh Roe and the Irish confederacy, but in 1600 he entered secret negotiations with leading royal soldier Henry Docwra.

The English government hoped to harness Niall's feud against Hugh Roe and promised him the lordship of Tyrconnell for his military assistance.

Niall's skills in guerrilla warfare emboldened the English troops and allowed Docwra to significantly weaken Irish forces in Ulster.

In early 1608 Niall instigated fellow spurned loyalist Cahir O'Doherty to launch a rebellion in Derry, but he was quickly implicated and put on trial.

[16] Hugh McManus had become senile in his old age,[17][18] and a violent succession dispute broke out amongst the greater O'Donnell family over who would succeed him.

[5][24][25] At the time Turlough was defending his lands from rival clansman Hugh O'Neill, Earl of Tyrone,[5] who had formed an alliance with the ruling O'Donnell branch.

Hugh Roe eventually escaped prison and returned to Tyrconnell in early 1592; Tyrone had bribed officials in Dublin to secure his release.

"[4][30] According to historian Terry Clavin, Hugh Roe was anxious to mend the divisions within the O'Donnell clan in order to make the Irish confederacy stronger.

Historian James O'Neill has theorised that Hugh Roe intentionally dispatched Niall to Belleek with the hope that he would die in the slaughter, thus easily eliminating a potential enemy.

[5] However, Niall was alienated when Hugh Roe deprived him of his castle of Lifford, which he had inherited from his father Conn.[7][5][4] In May 1600, English-born soldier Sir Henry Docwra established an English garrison in Derry.

[4][1] In September 1600, Hugh Roe left Ulster for a raid in Clare, leaving Niall in charge of besieging Docwra at Derry.

[39] Niall was one of many other disgruntled Gaelic Irish noblemen who would defect on Docwra's promises, including Arthur O'Neill[40] and Cahir O'Doherty.

[41] Accompanied by English forces, Niall marched to Lifford,[5] which had been left in the control of Hugh Roe's tanist and younger brother Rory.

[4] After the war, he wrote "I must confess a truth, all by the help and advice of Neal Garbh and his followers, and the other Irish that came in with Sir Arthur O'Neale, without whose intelligence and guidance little or nothing could have been done of ourselves".

[49] Niall's hold over Donegal virtually stopped Hugh Roe from entering Tyrconnell, and a subsequent month-long siege ensued.

By the time Irish forces were marching to Kinsale, Niall and Docwra had essentially conquered Tyrconnell and left Hugh Roe with no home to return to.

Additionally if Niall was disgraced, his land rights would be retracted by the Crown; this would allow Docwra to obtain a grant to parts of Tyrconnell.

[5] News of Niall's insubordination reached Mountjoy, who summoned him to Dublin with the apparent intention of granting him a patent of Tyrconnell.

[citation needed] In 1603, Caffar Óg O’Donnell and Mulmurry McSweeney Doe went to Tyrconnell, "with their people and cattle, to wage war with Niall Garbh and the English".

Niall ultimately gave himself up[5] and was allowed to proceed to London "to solicit pardon for his offences, and to obtain the reward for his service and aid to the crown of England.

[5] In March 1607, Niall served against Cathbhar Oge O'Donnell, and was reputed to have "got a blow in the service which he will hardly recover of long time, if he escape with his life".

[4][5][d] In September 1607, due to increasing hostility towards former confederacy members, Rory and Tyrone left Ireland for continental Europe.

[59][5] Niall rejected meetings with the English, starting bargaining with them over the price of his assistance, and demanded for him to be finally recognised as the rightful Lord of Tyrconnell.

[5] The Attorney-General for Ireland, Sir John Davies, found little difficulty in accumulating proof of Niall's correspondence with O'Doherty, but the question arose whether his guilt had not been condoned by his protection.

[5] "Pretending that he had more evidence to give for the king, but that he found the jury so weak with long fasting that they were not able to attend the service," Davies discharged them before they gave their verdict.

[65] He is described by Lughaidh Ó Cléirigh, Hugh Roe's biographer, as "a violent man, hasty, austere, since he was spiteful, vindictive, with the venom of a serpent, with the impetuosity of a lion.

[29] Historian Paul Walsh points out that Docwra would have described any Irishman of his time in this manner, and that because of Niall's betrayal of both Gaelic Ireland and the Crown, both Irish and English contemporary sources would be somewhat biased again him.

[67] Philip O'Sullivan Beare called Niall "a man of great spirit and daring, skilled in military matters".

[64] A report by Docwra describes Hugh Roe murdering Niall and Nuala's child in a furious reaction to his betrayal in 1600:[16][57] "O’Donnell hath of late hanged many of good account .

he dasht owt the brains of Neil Garve’s childe (of [four] yeares olde) againste a post, beinge in the mother’s custody, his owne naturall sister.