O'Byrne (Ó Broin), Sir John (d. 1367), lord of the O'Byrnes, was the son of Tadhg O'Byrne (c.1300–50), lord of the O'Byrnes. Nothing is known of his early life and little can be ascertained about his father, who ruled over the O'Byrnes during the 1340s and died about 1350. The death of Tadhg provided the Dublin government with a chance to interfere directly within the dynastic politics of the O'Byrnes. In autumn 1350 the justiciar, Thomas Rokeby (qv) (d. 1356), presided over John's election as chief after a campaign into O'Byrne territory in July. A favourite of Rokeby, John remained in English service until summer 1353, but he was determined to assert himself; this alarmed other Irish leaders, particularly Muircheartach MacMurrough (d. 1354), king of Leinster, who was so concerned by John's activities that he himself entered English service in 1353. In 1353–4 Muircheartach, Rory O'More (d. 1354/5), and Aodh O'Toole (qv) (d. 1376) all supported the government, contributing large forces to a successful expedition into John's country, that brought him to peace on 22 March 1354, when he surrendered ninety-two cows. By summer, the alliances had shifted and Muircheartach and John joined forces in a campaign against the English, providing ample opportunity for their respective dynastic rivals to prove their loyalty to the English. John's rivals included his cousin Murchadh O'Byrne (son of Maghnus), and the sons of Domhnall Óg MacMurrough (qv) (d. 1347), Domhnall Riabhach MacMurrough and Diarmait Láimhdhearg MacMurrough (qv) (d. 1369), who eagerly grasped their chance to attack their father's successor as king of Leinster. Muircheartach MacMurrough was quickly captured, but John defeated Rokeby, besieging him in Wicklow castle in October 1354. During the siege John requested Muircheartach's release of Rokeby, but Rokeby had Muircheartach brought by sea to Wicklow and executed him on the city walls as a warning to John – an act that shocked not only the Leinstermen but Irish kings countrywide.
John steadily undermined Rokeby's alliances among the Leinster Irish, attracting to his faction Aodh O'Toole and his brother John Ruadh O'Toole (qv) (d. 1388) who deserted the English and joined him some time after 9 August 1355. By the end of the year Muircheartach's son Art MacMurrough (qv) (d. 1362) had submitted, but John continued fighting throughout 1356, prolonging the disturbances in east Leinster; he finally submitted in 1357. War broke out again in May 1358, this time led by Art, John, and a new O'More leader – a dangerous confederation that the government tried unsuccessfully to neutralise by sending envoys to treat with O'More for peace. In July 1358 the O'Mores of Laois and Slemargy won a major victory over the English, prompting widespread Irish attacks upon the settlers throughout the Barrow valley, while in east Wicklow John and Aodh O'Toole besieged the castle of Killoughter. John was captured in August by James Butler (qv) (d. 1382), 2nd earl of Ormond, but was quickly released and resumed his campaign. In 1359 during a fierce offensive by Art and the O'Mores against Ormond's earldom, the earl, who was now justiciar, defeated them in a pitched battle in Laois. A general peace was made on 12 August 1359 and John voluntarily submitted at Carlow, where he was knighted.
The pressure on the settlers of the Barrow brought Lionel (qv) of Clarence (d. 1386) to Ireland on 15 September 1361. Despite an initial reverse inflicted by Sir John in Wicklow, Clarence shored up the English position along the Barrow by refortifying Carlow, and sealed the English commitment to the city by relocating the exchequer and the common bench there. He took captive Art MacMurrough and his tánaiste, Domhnall Riabhach, before campaigning separately against the O'Mores and the O'Byrnes. The MacMurroughs, led now by Diarmait Láimhdhearg, continued to threaten Carlow and the Barrow settlements. Sir John did not have as close a relationship with Diarmait Láimhdhearg as with Art, and he exploited the enmity between the MacMurroughs and the government, earning a fee for service against his erstwhile allies in 1365–6; at that time Tadhg O'Byrne and John Ruadh O'Toole were also rewarded with knighthoods. Sir John died shortly after this, causing a struggle for supremacy among the competing O'Byrne factions, which was eventually won by Sir John's first cousin Bran O'Byrne (qv) (d. 1378), son of Philip O'Byrne. During this conflict it is probable, though not certain, that Tadhg perished. If so it was his death that allowed Sir John's younger brother, Gerald O'Byrne (d. 1398), king of Uí Faoláin, to assume the leadership of the faction headed by their family. He played the role of kingmaker in the continuing struggle, supporting Bran son of Philip, and helping him to win supremacy about 1376. Gerald's insight served him well, for he succeeded Bran son of Philip as king of Uí Faoláin in 1378.
Throughout his career Gerald was allied closely with his cousin Art Mór MacMurrough (qv) (d. 1416/17). In 1386–8 the pair jointly earned £48 14s. for keeping the Irish of Leinster at peace, and in 1392 Gerald and Feidhlim O'Toole (qv) (d. 1404), with Art Mór and the O'Mores, ransacked the English parts of Carlow. Gerald's close relationship with Art Mór made the O'Byrnes the target of major government expeditions into their country during 1393 and again in May 1394. In October 1394 Richard II (qv) landed at Waterford to begin the reform of his lordship in Ireland. The combined strength of the king's army and the forces of James Butler (qv) (d. 1405), 3rd earl of Ormond, brought Art Mór and the O'Byrnes to submission by 30 October. At Ormond's insistence Art Mór was imprisoned, but was released in return for Gerald, John O'Nolan, and Maoilsheachlainn O'More, who were briefly confined at Dublin. On 7 January 1395 at a ceremony held near Tullow, Art Mór and Gerald promised to evacuate Leinster and conquer fresh lands as royal mercenaries; on behalf of the Leinster Irish they both affixed their seals to the agreement with Richard, which suggests that they considered each other equals.
The peace was broken by Ormond, who sought to resurrect old claims to lands within Irish kingdoms, including Wicklow; as soon as 18 January 1395 his aggression caused Gerald to complain to the king. But Gerald and Art Mór resisted Ormond's provocations and at a meeting on 16 February with Richard's officials at Balgory near Carlow pledged to forfeit 20,000 marks each if the pact of 7 January were broken. Once Richard left for England in May 1395, his treaty with the Irish came under pressure from the English of Ireland, though the peace held until 1396 and Art Mór, Gerald, and other Leinstermen were faithful to the agreement. In summer that year Gerald's cousin Donnchadh O'Byrne (qv), son of Bran, of Newrath (d. 1434) fought for the king in France – evidence of continued goodwill between Leinster and its overlord.
The appointment of Richard's heir, Roger Mortimer (qv) (d. 1398), earl of March, as lord lieutenant of Ireland was the catalyst for a new crisis. Mortimer behaved more like a great magnate than Richard's representative, campaigning vigorously against the Irish, and forming alliances with Ormond and Gerald fitz Maurice FitzGerald (qv) (d. 1432), 5th earl of Kildare. Incursions by Ormond and Mortimer into the Leinster mountains brought the crisis to the boil. In summer 1396 Feidhlim O'Toole revolted, but under Gerald's leadership the O'Byrnes continued to observe the peace, until in early 1397 Mortimer, supported by Ormond, sacked Gerald's stronghold and burned his country, marking the occasion by knighting seven of his companions. In the face of such provocation, the ageing Gerald, by now no more than the figurehead of the O'Byrnes, authorised Donnchadh son of Bran and Domhnall O'Byrne to retaliate and soon Ormond's lands near Arklow and Carlow were burning. On 20 July 1398 Gerald had his revenge upon the hated Mortimer and his allies, when Donnchadh, Domhnall, the O'Tooles, and some of Art Mór's troops cornered Mortimer at Kellistown in Carlow and killed him. But within weeks of this victory Gerald was dead, leaving Donnchadh and Domhnall to struggle over the succession.