[2] His father's family, the O'Briens, were a Gaelic Irish dynasty that descended from Brian Boru, medieval high king of Ireland.

In 1584 he was one of the commissioners who established the agreement that tanistry and the law of partible succession should be abolished in Connaught, and a tax of ten shillings a quarter be paid on land.

Donogh and Elizabeth had two sons: His second wife died on 12 January 1617. in 1595 Tyrone's rebellion, also called the Nine Years' War, broke out.

In command of a large force, he passed the River Erne in July and invaded Hugh Roe O'Donnell's country, but retreated in August when a truce was signed.

In September he was detached by Sir William Russell, Lord Deputy of Ireland since 16 May 1594, with five companies of foot and 145 horse, for the defence of Newry.

[24][25] He therefore was absent at the Battle of Yellow Ford in August 1698,[26] where Hugh O'Neill, Earl of Tyrone defeated and mortally wounded Sir Henry Bagenal, marshal of the Royal Irish Army.

[28] Thomond returned from England, and after spending three months with his kinsman, the Thomas Butler, 10th Earl of Ormond, in collecting forces, he invaded Clare to revenge his brother's imprisonment and recover his possessions.

He procured ordnance from Limerick, and laid siege to the castles that resisted, capturing them after a few days' fighting; at Dunbeg, which surrendered immediately, he hanged the garrison in couples on trees.

During the rest of 1599 Thomond accompanied Robert Devereux, 2nd Earl of Essex on his progress through Munster, but left him at Dungarvan and returned to Limerick, being appointed governor of Clare on 15 August, and made a member of the privy council on 22 September.

Next year, after holding an assize at Limerick in February, at which sixteen men were hanged, Thomond again went to England, probably with the object of obtaining the governorship of Connaught and of securing the union of Clare with Munster.

He then again visited England, and, as a recompense for his services, his request for the transfer of Clare was granted, though the lord-deputy and privy council of Ireland were opposed to the measure.

[37] Pollard (1895b) concludes that he was one of the most influential and vigorous of the Irish loyalists; and, though his devotion and motives were sometimes suspected, Carew wrote that "his services hath proceeded out of a true nobleness of mind and from no great encouragement received" from the court.