O’Byrne, Aodh (c.1596–p.1652), soldier, was the second son of Phelim O'Byrne (qv) (d. 1631), lord of Crioch Raghnuill, and Una O'Toole (d. 1628), daughter of Fiach O'Toole (qv) (d. 1578), lord of Fartry and Ferter. Their father favoured Aodh over his elder brother, Brian O'Byrne (qv) (d. 1652), which led to vicious rivalry between them. After submitting before the Dublin council in March 1601, Phelim lived the life of a dutiful subject, fulfilling his pledge by serving as a justice of the peace and sitting in the Irish parliament, and he encouraged his sons to conform outwardly by adopting English custom, language, and dress; in pursuit of this policy, he may have dispatched the young Aodh to England to be educated, as he is recorded there in 1614. On his return from England some time in the middle of the 1610s, Aodh entered service as an enforcer of the law. In November 1619 he was rewarded with 100 livres for killing robbers in Leinster, and on the orders of Lord Deputy St John (qv) on 18 June 1620 he took twenty footmen and four horsemen to bring in Morrys MacEdmund Kavanagh. About 1622 he arrested Sean Ban mac Tadhg and Toirdhealbhach Archbold, associates of the rebel Murchadh Bacach Kavanagh, bringing them to Wicklow for trial and execution.
Meanwhile the O'Byrne family was engaged in bitter legal disputes with the New English over possession of the Cosha region, resulting in low-level continuing violence; Aodh's own lands were raided by the foster brother of Sean Ban mac Tadhg at the instigation of Laurence Lord Esmond (qv), the latter, with Sir Henry Bellings, having designs on Aodh's lands at Tomekowly and Ballynatombeha. The chief threat, however, came from the lord deputy, Henry Cary (qv), Viscount Falkland, who was determined to make the ‘O'Byrne country’ available for plantation. His initial efforts, which involved having the crown's title found by inquisition in 1623, were thwarted by his rivals at the English court. A fresh opportunity rose in 1625 when Phelim, as justice of the peace, tried and hanged the murderers of a land speculator, the Rev. Robert Ponte of Rathdrum, a fellow justice of the peace. As it was well known that Phelim and Ponte hated each other, it was rumoured that Phelim himself had authorised Ponte's murder. Phelim's brother and enemy Reamain O'Byrne (d. in or after 1642) plotted with Cathaoir O'Byrne of Knockrath and Esmonde to accuse Phelim and his sons of the murder. Together with Falkland, they gathered or fabricated evidence to support their case. One witness alleged that Aodh was the inventor of the plot to murder Ponte, and another spoke of hearing Phelim and Aodh discussing the murder of Ponte at Phelim's residence of Ballinacor; Margaret Ponte, daughter of the deceased, later said that Aodh himself had committed the crime. A commission was also established to investigate Phelim's title to Crioch Raghnuill, resulting in many of Falkland's friends acquiring patents to various lands there in June 1628. In August 1628 Phelim and his sons were indicted of Ponte's murder by a grand jury composed of their enemies at the sessions of the Wicklow assizes and were committed to Dublin castle to await trial before the king's bench. In the same month Aodh travelled to court with his brother-in-law, John Wolverston, whose son Edward acted as agent for the recently acquired Wicklow interests of the influential earl of Carlisle. By a stroke of fortune, the assassination of the duke of Buckingham deprived Falkland of his patron, Carlisle's influence prevailed and Aodh's petition was favourably received. In December 1628, on the recommendation of the commissioners for Irish affairs, the release of the O'Byrnes was ordered and their tenure of their lands was confirmed at an increased rent. Falkland's protests were unavailing and enquiries into his treatment of the O'Byrnes contributed to his recall in August 1629.
In early 1629 Aodh took ship for Spanish Flanders, where, in Brussels, he joined the regiment of the earl of Tyrone. He raised a company of Irishmen at the town of Diest and entered the service of the infanta Isabella as part of Tyrone's regiment. However, he found it hard to maintain himself and his men and on 16 October he wrote to Isabella requesting her urgent attention to the fact that his men were without quarters and the winter was upon them. On 9 December the infanta ordered the governor of Bourbourg to pay, arm, and house Aodh and his men on 21 December, and on 1 January 1630 Aodh was rewarded for his efforts with a grant of 30 crowns. With a company he had raised in Bourbourg, he was stationed in Artois and Dunkirk in northern France 1630–31. In 1634 he returned to Ireland for three months to recruit men for the Spanish service, during which time he took possession of the lands conveyed to him in his father's will of 1630. He seems to have persuaded his brother Art to join him in the Spanish service – the two brothers returned together to Ireland in 1636 as recruiting captains. They fought with distinction on Spanish campaigns in France, as well as serving against the Catalan rebels in 1640.
As Spain declined, Aodh became disillusioned and resigned his commission. In June 1641 he famously asked Owen Roe O'Neill (qv) whether ‘we are to adventure our lives for the succouring of a scabbed town of the King of Spain's where we may happily lose our lives and we can expect no worse than death if we go into our own country’ (J. T. Gilbert, Contemporary history (1879), i, pt 1, 396–7). He re-joined the Spanish army and travelled to England where he sought and received permission, on 9 July, to recruit Irish troops, ostensibly for Spanish service. In Ireland in August he became involved in the ‘colonels’ plot’ to overthrow the government on the king's behalf and in September was instrumental in persuading some of the colonels to combine with the group of Ulster conspirators, led by Sir Phelim O'Neill (qv), who were already in touch with Owen Roe. Aodh was one of those who were assigned to take part in the attack on Dublin castle on 23 October and also one of those who evaded capture when the plot was betrayed. He was one of the leaders of the Irish army which routed the Drogheda relief force at Julianstown on 29 November 1641 and he played a prominent part at the meeting some days later at the hill of Crofty at which the Old English of the Pale agreed to make common cause with the northern rebels. For some months thereafter, he commanded a garrison at Kilsalchen, seven miles north-west of Dublin, which became the first target of the English reinforcements which reached the area in February. Aodh withdrew to the midlands and after Drogheda was relieved he was one of many commanders of the large rebel army that met with decisive defeat when it forced an engagement with the earl of Ormond (qv) at Kilrush on 15 April. Among the dead were one of his brothers and two of his cousins (both sons of Reamain O'Byrne).
Notwithstanding this disaster Aodh's considerable military experience propelled him up the ranks of the emerging army of the Catholic Confederation. A shrewd man, he was also more affable than his brother Brian and gained the ear of the Old English who largely controlled the process of organisation. When it was decided, on 7 June 1642, to supplement the existing local military forces by forming a ‘running army’ of 4,000 foot and 500 horse, Aodh was appointed its commander with the rank of lieutenant general. This prominence was shortlived. The return of exiled officers from continental armies in the summer of 1642 and the related decision to organise the confederate armed forces on a provincial basis reduced Aodh's status sharply. The ‘running army’ ceased to exist and Thomas Preston (qv) was appointed general of the Leinster army. Though Aodh was nominally his second-in-command, his responsibilities were normally confined to Wicklow, with the notable exception of the battle of Ballybeg in March 1643 when he provided the cover that enabled Preston to escape being routed by Ormond's army. He occasionally campaigned outside Leinster, serving in Ulster during 1644 and Munster two years later.
As the confederates in Wicklow came under more pressure from the English, the supreme council of the confederacy on 15 November 1646 ordered the catholic gentry there to maintain them, a move that Brian O'Byrne and others opposed and finally quashed. The Wicklow gentry also opposed Aodh's forthcoming campaign with Preston, complaining that the county would be exposed; they gained the support of the Leinster committee of the confederacy, which countermanded Preston's order. Preston, however, persisted and Aodh was forced to join him at Carlow, which was captured on 2 June; this initial success was overturned at the battle of Dungan's Hill on 8 August 1647, when the parliamentary forces under Colonel Michael Jones (qv) wiped out the Leinster army. Aodh was captured and taken to Dublin; on 27 September the government, describing him as a ‘very dangerous rebel’, urged that he be sent to England for trial, but this was not done. The parliamentarians carried fire and sword into Wicklow; in response the confederacy sent forces under Colonel Richard Butler, but they only caused more misery, every ‘village, montaine, and cottage cryinge after them, more pittifully then after the enemy’. Brian O'Byrne now consolidated his links to O'Neill and the papal nuncio GianBattista Rinuccini (qv), allowing O'Neill to camp his army in Wicklow during winter 1647. In September 1648 O'Neill exchanged Theophilus Jones (qv), whom he held prisoner, for Aodh, presumably expecting that this, their old alliance in the service of the Spanish, and Aodh's past service to him as an intermediary with Ormond in 1644 would secure Aodh's loyalty. However, O'Neill's connection with Brian O'Byrne was more than Aodh could accept, and he turned instead to Ormond, who appointed him governor of Wicklow on 20 September. In this role Aodh worked to subdue his brother, strengthening the defences of the castles of Carnew and Arklow. In early 1649 Brian, with some highland troops of Randall MacDonnell (qv), marquis of Antrim, attacked Aodh, though he was still able in March to bring his regiment to Tullow to aid the earl of Castlehaven (qv). But the situation in Wicklow deteriorated and on 17 March 1649 Aodh wrote to Ormond: ‘my brother Bryen hath about 500 foot of the Highlanders and as many more loose fellowes which adhere unto him and both point their malice chiefly at me’.
The arrival of Oliver Cromwell (qv) in Ireland on 15 August transformed the situation, uniting the different factions of the anti-parliamentarian forces. On 20 October 1649 Brian's position was recognised in a treaty between O'Neill and Ormond, and the brothers made common cause against Cromwell's army in Wicklow, ambushing his troops in mountain passes and destroying Powerscourt castle. In November and December they besieged Arklow, which had fallen to the parliament in September, but were forced to retreat before a large parliamentarian force and were defeated in January 1650 when they renewed the siege. They recovered to control large parts of Wicklow and Wexford and in March 1652 scored a victory against English raiders in Glenmalure, killing 300 of them. In the spring of that year they carried out raids at Arklow, Castlesallagh, and close to Dublin, and when the Leinster army finally surrendered on 12 May 1652 neither O'Byrne was admitted to the terms of the surrender and a reward of £400 was placed on each of their heads.
Support for the O'Byrnes now wavered. In July Edmund Ludlow (qv) with an army of 4,000 marched against them and the Kavanaghs, laying waste to the crops in Wicklow and Wexford, and offering protection to those who would submit; their forces deserted the brothers and they fled. Brian was probably killed by the English on the island of Inishbofin on 15 December 1652. Aodh disguised himself as a ‘poore common souldier’ and took ship ‘for some Catholicke countrie’. His fate is unknown.