O'Neill, Hugh (Aodh Dubh Ó Néill) (c.1605–c.1660), officer in the Spanish army, the son of Art Og O'Neill and the nephew of Hugh O'Neill (qv), 2nd earl of Tyrone, was born in exile in the Spanish Netherlands, and like so many of his class joined the Spanish army. He served in the regiment of another uncle, Owen Roe O'Neill (qv), and distinguished himself in several battles, earning a reputation as a brave and courageous officer. In 1642 he and Owen Roe sailed with other Irish émigrés out of Dunkirk for Ireland, landing at Killybegs, Co. Donegal. Aodh Dubh was captured by English forces in Co. Monaghan in 1643 and remained in captivity until the victory of Owen Roe over the Scots at Benburb in 1646; Owen Roe traded hostages to gain the freedom of Aodh Dubh and other experienced catholic officers held captive. Once free, Aodh Dubh was promoted to the rank of major general and entrusted with considerable responsibility. After his uncle's death on 6 November 1649, Aodh Dubh put himself forward for election to the position of commander-in-chief of the Ulster army. Although he was unsuccessful, he enjoyed the confidence of the royalist James Butler (qv), marquess of Ormond, and in February 1650 Ormond appointed him governor of Clonmel with instructions to defend the town against the advancing forces of Oliver Cromwell (qv).
Up to then Cromwell had enjoyed impressive though bloody success throughout Leinster. On 27 April he subjected Clonmel to a frontal attack but was kept at bay by Aodh Dubh's garrison, which held out until 9 May; on that day a determined cannonade breached the walls, but when Cromwell's troops entered through the breach they met a second line of defence, and under a hail of bullets they were forced to retreat in disarray. Aodh Dubh then resolved to save his troops and quit the town; at dead of night they stole away towards Waterford, leaving the mayor of Clonmel to sue for peace. On 10 May the mayor dispatched his envoys to Cromwell's camp to seek terms and quarter. In ignorance of Aodh Dubh's escape, Cromwell readily agreed, but when he found that he had been tricked he sent Lieutenant-general Edmund Ludlow (qv) and his cavalry in pursuit of Aodh Dubh; they caught up with the escaping column and attacked it savagely, killing 200 men, though Aodh Dubh managed to escape.
The royalist–confederate cause was effectively lost when the Ulster army was routed in June at Scarrifhollis, but Aodh Dubh made his way to Limerick, the last bastion of the king's support in Ireland. On 19 June 1651 Cromwell's son-in-law Major-general Henry Ireton (qv) appeared before Limerick and called for its surrender. When Aodh Dubh refused, Ireton's artillery rained down upon the town, destroying the castle the following day. But Limerick held out and as a result Aodh Dubh was excluded from any form of quarter. On 29 October, weakened by hunger, plague, and the bombardment, the people of Limerick finally surrendered to Ireton; more than 5,000 inhabitants and 800 of the garrison had lost their lives during the siege. As Aodh Dubh marched out of the city with his remaining 1,200 men some of them were seen to fall dead of the plague. Ireton immediately hanged General Purcell but he spared the life of Aodh Dubh; at a council of war the next day Aodh Dubh was condemned to execution, but the death sentence was revoked on 1 November in response to his appeal on the grounds that he had only performed his duty as a soldier. Aodh Dubh was sent to the Tower of London to await his fate, arriving there on 10 January 1652. In July the Spanish ambassador appealed for his freedom because he was a subject of the Spanish king, and claimed that his release would encourage troublesome Irish soldiers to leave Ireland and enter the Spanish service. His plea was granted, and Aodh Dubh took ship for Spain. He is last heard of in October 1660 when he wrote from Madrid to Ormond and Charles II, enquiring about his rights to the then vacant earldom of Tyrone.