O'Byrne (Ó Broin), Donnchadh (d. 1434), lord of the O'Byrnes, was the son of Bran O'Byrne (d. 1378), king of Uí Faoláin, son of Philip O'Byrne and probably Dearbhail O'Toole. Bran became lord of the O'Byrnes upon the death of his first cousin Sir John O'Byrne (qv) about 1367, and though he was a powerful soldier he lacked diplomacy when dealing with his vassals. On 27 May 1371 he concluded an agreement with Sir William Windsor (qv), which suggests internecine conflict among the O'Byrne families; by its terms he promised never to obey any MacMurrough king of Leinster, to rebuild the church of Wicklow, and to acknowledge the rights of the archbishop of Dublin. Unsurprisingly, once Bran had silenced his rivals, the agreement was disregarded: in 1374 he captured and demolished the royal castles of Wicklow and Newcastle; though the English promptly retook and rebuilt them, Bran captured them again in 1376, and briefly seized Kindlestown castle in 1377. By the time of his death in 1378, it is clear that he had formally adopted the title of king of Uí Faoláin, proclaiming that his power now ran from the outskirts of Bray to Tullow. Bran's successes were accompanied by the reemergence of good relations between the O'Byrnes and the MacMurroughs. After the killing of Donnchadh MacMurrough in October 1375, his nephew Art Mór MacMurrough (qv), whose mother was Bran's first cousin, went to great lengths to cultivate good relations with the O'Byrnes. His efforts were rewarded by the continuous goodwill and military support of Gerald O'Byrne (qv) (d. 1398), Domhnall O'Byrne, and Donnchadh O'Byrne.
Nothing substantive is known of Donnchadh's youth and early manhood except that he lived at Newrath in Co.Wicklow and married as his first wife Honora O'Toole. And for most of his early twenties he seems to have played second fiddle to Gerald O'Byrne, Bran's successor, as tánaiste of the O'Byrnes. His first appearance of note in the historical record occurs in 1394–5, when the O'Byrnes demonstrated their support for Art Mór during the expedition to Ireland of Richard II (qv). Gerald was given as a hostage by Art Mór during his submission to Richard in October 1394; his imprisonment was short-lived, but his absence may have encouraged Donnchadh to assert his independence among the O'Byrnes, and he travelled to Dublin castle to make a personal submission before the king. However, Gerald's position as Art Mór's leading vassal was confirmed on 8 January 1395 by his presence at the sealing of MacMurrough's agreement with Richard; the two affixed their seals to the parchment, pledging 20,000 marks if they failed to evacuate their Leinster homelands and conquer fresh lands as royal mercenaries. The peace was soon jeopardised by the English magnates of Leinster, who sought to reclaim lands long lost to the Irish, though it held until 1396. In February of that year Donnchadh visited Richard and in August he served with Tadhg Aibhle O'Carroll (qv) of Ely against the French at Calais, which proved the widespread commitment of the Leinster Irish to the agreement.
The aggressive strategy pursued by James Butler (qv), 3rd earl of Ormond, and the lord lieutenant Roger Mortimer (qv) in the Leinster mountains threw the peace process into chaos. By summer 1396 Feidhlim O'Toole (qv) started again to harry the settlers and the O'Byrnes followed suit in early 1397. The crisis in Leinster came to a head when the O'Byrnes killed Mortimer at Kellistown on 20 July 1398; Richard revoked the agreement and resolved to mount a fresh campaign against Art Mór. At about the same time Gerald died, leaving Donnchadh and Domhnall O'Byrne of Glenmalure to dispute the succession, though their feud was quickly shelved when Richard landed at Waterford on 2 June 1399. Together they went to the support of Art Mór, who, after an initial defeat at Richard's hands, dealt his army a crippling blow in the O'Byrne lands of the Leinster mountains; the French chronicler Jean Creton who was on this expedition described how Art Mór with Donnchadh and the O'Tooles used the protection of the dense mountain forests to harry the flanks of Richard's demoralised troops. The expedition ended in riots as starving troops waded out to supply ships off the coast near Arklow in search of bread. At a famous parley, Art Mór and Domhnall denounced Richard, rejected peace, and proclaimed the Leinster king as the true king of Ireland.
The landing of Henry Bolingbroke at Ravenspur forced Richard to embark for England on 29 July 1399, where he was deposed and replaced by Bolingbroke as Henry IV. In Ireland Art Mór and the O'Byrnes held to their pact to pursue the war. In 1400, strengthened by O'Meagher mercenaries from Ikerrin in north Tipperary, Domhnall took the royal castle of Newcastle. But in July–August 1401 his mercenary force, with its captain Tadhg O'Meagher, was slaughtered by the Dublin citizens at Bloody Bank near Bray. Domhnall submitted to Thomas of Lancaster (qv), the new lord lieutenant, on 8 November 1401, but shortly afterwards he either died or was deposed by Donnchadh. Donnchadh also allied himself with the O'Meaghers, taking Siobhán O'Meagher as his second wife, and emulated Domhnalls' attempts to conquer north Wicklow. He took Newcastle in 1405 (which remained in O'Byrne hands until 1542), a feat that heralded a period of expansion and stability for the O'Byrnes; for the first fifteen years of the new century, the strength of the O'Byrne–MacMurrough alliance brought Gaelic Leinster both new territories and military power. But during Art Mór's last years, his kingship declined: his sons lacked his abilities and an alliance was made with the Butlers through the marriage in 1416–17 of Art Mór's heir, Donnchadh MacMurrough (qv) (d. 1478), to Aveline Butler, the half-sister of James Butler (qv), 4th earl of Ormond. The effects of improved relations with the Butlers were reflected throughout Leinster. In this context, given the nascent Talbot–Ormond feud, Art Mór's 1416 devastation of the liberty of Wexford belonging to Gilbert Talbot can be understood. To the north Donnchadh took Wicklow castle with the aid of Ormond's governor of Arklow, killing Talbot's constable. In response Talbot reasserted himself against the O'Byrnes and the O'Tooles, forcing their temporary submission.
After the death in 1416 or 1417 of Art Mor, the greatest medieval king of Gaelic Leinster, Donnchadh aspired to become the most powerful Irish leader in Leinster. This ambition contributed to a deterioration of relations between him and his son-in-law, Gerald son of Art MacMurrough after 1423. On 10 April 1425 Donnchadh made a pact with the justiciar John Talbot (qv), recognising the jurisdiction of the archbishop of Dublin, Richard Talbot (qv), within his domains and agreeing to protect the justiciar's tenants within the liberty of Wexford – a significant challenge to the MacMurroughs. Within three weeks, however, Ormond had replaced Talbot, and it was several months before he turned to the affairs of the Leinster Irish: Diarmait O'Toole made his submission to Ormond on 8 August and Donnchadh not until 6 December. No military pressure was brought to bear upon the O'Byrnes and there was a special degree of trust and cordiality in Donnchadh's submission to Ormond: Donnchadh declared himself not only a liege subject of Henry V but also Ormond's man, promising to protect merchants entering his lordship; in return Ormond promised that as long as the O'Byrnes observed the terms of the peace, they were entitled to his protection. By 1427 this alliance with the Butlers seems to have eased relations between Donnchadh and Gerald son of Art.
The rift between the O'Byrnes and the MacMurroughs had been patched up, but the return of Ormond's brother-in-law, the newly freed Donnchadh son of Art MacMurrough, to Ireland brought a change in Ormond's policy in Leinster. Now he was anxious that Donnchadh son of Art would regain his former hegemony over the MacMurroughs and their allies. And to aid Donnchadh son of Art's bid to regain his kingdom, the English granted him a fee in the hope that he would be strong enough to curb Gerald son of Art and O'Byrne. While Gerald son of Art lived, Donnchadh son of Art did not pose a substantial threat to Ormond's new alliance with O'Byrne. When Gerald son of Art died in 1431, rifts opened with the O'Byrnes as Donnchadh son of Art regained his inheritance.
Donnchadh O'Byrne died in 1434, and his successor, his brother Edmund O'Byrne (d. 1446), remained alienated from the MacMurroughs and their Butler allies. From the late 1430s, he actively lent military aid to such enemies of the Butlers as Thomas FitzGerald (qv) (d. 1478), later 7th earl of Kildare, and seems to have encouraged his nephews, the sons of Gerald son of Art, to challenge Donnchadh son of Art's control of MacMurrough territory. In 1446 war broke out between the O'Byrnes and the MacMurroughs, in which the O'Byrnes prevailed, killing a prominent MacMurrough noble and raiding Uí Cheinnsealaigh by sea; moreover in the winter of that year John Talbot extracted submission from Donnchadh son of Art MacMurrough, but made no such demand of Edmund O'Byrne. But O'Byrne ascendancy in Leinster was short-lived: Edmund died in 1446, and was succeeded by his cousin Dúnlaing son of Gerald O'Byrne.