Parents Conchobhar mac Toirdhealbaig Ó Briain | ||
Sir Donnell O'Brien was a Gaelic leader from Thomond in the Kingdom of Ireland. The brother of Donough O'Brien, 2nd Earl of Thomond he challenged the succession of his nephew Connor O'Brien, 3rd Earl of Thomond to the Lordship of the O'Brien clan. By traditional Gaelic practice O'Donnell was inaugurated as "The O'Brien", but his attempts to gain Crown recognition of this failed. He was unsuccessful in his attempts to be made an Earl, but he was given a knighthood as a sign of royal favour.
Sir Donnell's refusal to accept the authority of his nephew led the Lord Lieutenant of Ireland the Earl of Sussex to bring Crown forces into County Clare to support the Earl. Following the departure of Sussex's forces Sir Donnell defeated the Earl and his allies at the Battle of Spancel Hill in 1559. The following year Crown forces returned again to support the Earl. Fighting continued in the area until a compromise agreement was reached in which the Earl would be confirmed in the succession, but Sir Donnell would be given independent status.