O'Byrne, Seán (c.1470–1551), eldest son of Reamain O'Byrne (qv) (c.1450–1510), became 7th lord of Crioch Raghnuill on his father's death. It seems, though, that the Fitzgeralds of Kildare punished him for his father's resistance by fostering the ambitions of his distant cousin Domhnall mac Sheaain Glais O'Byrne of Knockrath. In 1526 this Domhnall displayed a special relationship with Gerald Fitzgerald (qv) (d. 1534), 9th earl of Kildare, covenanting to pay to the earl's collectors 4d. for every cow in Knockrath. The underlying condition of the agreement was that if Domhnall ever warred on the earl, he was to forfeit his lands forever.
In the early 1530s, though, Seán outpaced his cousins of Knockrath and reasserted his lordship over them. Central to the resurrection of his power as the lord of Crioch Raghnuill were a number of factors. Seán married well, which no doubt helped his cause; he married first Doireann, daughter of Bran mac Taidhg O'Byrne of Cronroe. His second wife was Una, daughter of Alexander MacDonnell (d. p.1556), captain of the 9th earl of Kildare's galloglass. Seán's most important alliance, though, was with the highly ambitious Toirdhealbhach O'Toole (qv) and his younger brother Art Óg Ó'Toole (qv) (d. c.1546). What first propelled Seán to prominence was his support of Toirdhealbhach's exploitation of the collapse of the Kildare hegmony in 1535. This alliance, combined with their links with the young Domhnall MacMurrough Kavanagh (qv) (d. c.1542) of Garryhill and the Art Boy Kavanaghs of the Blackstairs mountains, further destabilised east Leinster. Indeed, the politics of the Irish of east Leinster after 1535 were increasingly dominated by this loose confederation of traditionally junior families.
Inevitably this led, in May 1539 and June 1540, to major government campaigns against them. But a four-week campaign by Sir Anthony St Leger (qv) and James Butler (qv) (d. 1546), 9th earl of Ormond, finally forced Toirdhealbhach and his O'Byrne allies to submit in November 1540. After Toirdhealbhach was dispatched to England in April 1542 to make his submission to Henry VIII, Seán reappeared in July that year, when Tadhg mac Gerald O'Byrne of Kiltimon and his leading nobles, including Seán (referred to as ‘John fitz Redmund’ in the official report), agreed (4 July 1542) to the gradual introduction of English rule into their lordship as part of the process of surrender and regrant. On the face of it, the agreement allowed the O'Byrnes a great deal of autonomy over their own affairs. But, as time proved, the reality was to be otherwise. In fact, the agreement, combined with the political fragmentation of the O'Byrne lordship, paved the way for its replacement by an English seneschalcy.
Peace proved short-lived, as strife among the O'Tooles again destabilised the Irish of east Leinster. In November 1542 Toirdhealbhach forcibly attempted to exert his rights within the territory of his cousin, Toirdhealbhach mac Seaain O'Toole of Imaal, but he was killed in a surprise morning attack. Toirdhealbhach's death was to open up considerable opportunities for Seán and his son, Aodh O'Byrne (qv) (d. 1579). After 1542 they positioned themselves to fill Toirdhealbhach's leadership role among the Irish of the Leinster mountains, and this led them into direct conflict with the Butlers of Ormond. The Butlers planned to extend their authority into the Wicklow district known as Cosha, lying to the north-east of Shillelagh, a territory where all the senior O'Byrne branches held lands. By doing so, the Butlers were resurrecting claims dating from the thirteenth century. However, Seán and Aodh had also moved into Cosha and seized a large slice of this territory, centred in the parishes of Kilcommon and Preban, near Aughrim and Tinahely. This posed a major obstacle to the Butler advance. Their expansion also irritated the senior O'Byrne families who were allied to the Butlers. Thus in 1543 the Butlers decided to challenge those territorial gains, focusing particularly on Preban and Kilcommon. In October 1543 Sir John Travers (d. 1561), a Palesman, was appointed as constable of Arklow with instructions to enforce old Butler claims to these lands. Although Travers failed in this objective, his attacks on the O'Byrnes of Glenmalure, combined with separate forays against some of the O'Tooles and the MacMurrough Kavanaghs, seriously disturbed the Irish leaders within the region.
But the ambitions of the O'Byrnes of Glenmalure were to grow. Having withstood the Butler assault of 1543, they now saw themselves as the emerging regional power, and, instead of remaining content with their relatively minor position, they sought to expand their influence southwards by interfering among the MacMurrough Kavanaghs. In 1545, though, they received a serious check when Cathaoir MacMurrough Kavanagh (qv) (d. c.1554) defeated them and their kinsman Gerald mac Cathaoir Kavanagh (qv) (d. c.1549) of Garryhill in a pitched battle near Hacketstown. However, the safety of the forested and hilly fastness of their homeland allowed Seán and Aodh to recover and to rethink their strategy.
In spite of their reverse at Hacketstown, they soon reemerged from Glenmalure to begin during late 1545 a lucrative business in levying protection money from the English settlements located amid the foothills of the western and northern Leinster mountains. This upsurge in violence was paralleled by the attacks made by Brian O'Connor Faly (qv) (d. c.1560) and Giollapádraig O'More in late 1546 on the settlers in the midlands. These attacks coincided with the recall of St Leger to England in the spring of 1546 to answer charges. Before his departure, he appointed as lord justice Sir William Brabazon (qv) to govern Ireland in his absence. Despite St Leger's declaration that the O'Byrnes and the O'Tooles were spent forces, clearly there had been a dramatic increase in the military strength of the O'Byrnes of Glenmalure.
The rise of Seán and his son Aodh had its origins in the political turmoil sweeping west Leinster and the midlands. If the violent imposition of the plantations profoundly disturbed the Irish of west Leinster and the midlands, they also affected east Leinster and the rest of Ireland. The fighting in west Leinster and the midlands was irrevocably to redefine Irish politics. Through the harsh application of martial law throughout Leinster, combined with the dispossession and execution of various Irish lords, a seething hatred arose among the Leinstermen towards the government. The effect of the plantations on the Leinster chessboard was a decisive eastward shift in the provincial centre of gravity to its edge, the Wicklow mountains.
From there Seán and Aodh offered refuge to the dispossessed in their virtually impenetrable territory of Glenmalure, hoping to harness new military manpower for their own purposes, as well as advancing their status through the patronage of poets. Their rise was also helped also by the death of Art Óg O'Toole of Castlekevin (a. November 1546), which left them alone to pose as the leaders of the mountain lords. Their incomes from extortion and timber enabled them to buy firearms. An indication of the growth of Seán's importance is that Lord Justice Brabazon led an expedition against them between 1 April and December 1546, which they defied successfully.
After the collapse of the O'Tooles of Imaal and the Fitzgerald rebels in the summer of 1547, Seán again sought to spread his influence into the power vacuum in west Wicklow and east Kildare. To the south, the O'Byrnes now began to consolidate their hold on their threatened lands of Kilpipe and Preban. Their activities did not go unnoticed by the Butlers, who in 1548 moved to halt the development of Seán's hegemony over the southern and central Wickow mountains; this proved a major factor in the growing destabilisation of the Irish of east Leinster. The Butlers justified their assault by pointing to their claims to long-lost lands lying within Crioch Raghnuill, Seán's lordship. Although the details of this latest Butler offensive remain obscure, clearly the lords of Glenmalure were obdurate and vehemently resisted this intrusion.
In July 1548 the Butlers unleashed Tibbot Walsh, constable of Arklow, and the O'Byrnes of Newrath. Their invasion of Crioch Raghnuill proved successful as their troops were quartered throughout the territory, forcing Seán and Aodh to retreat, along with their cattle, higher into the Wicklow uplands. Facing defeat, Seán dispatched a letter begging protection from Sir Edward Bellingham (qv), the lord deputy. In response Bellingham intervened with the Butlers and their allies, depriving them of victory. By September 1548 Seán had recovered, benefiting from this breathing space, and authorised Aodh to begin a fresh assault on the Pale. Seán and Aodh changed their tactics, beginning a double game with the English. While the son remained aloof from their overtures, the father, diplomatically, concluded a peace at Dublin during October 1548. This was the last time that Seán would play an active role in the government of Crioch Raghnuill, leaving that to the extremely wily Aodh. Seán was last mentioned with his wife Una in his pardon of 1550; he died shortly afterwards. He had four sons and a daughter by his two wives; Aodh succeeded him as 8th lord of Crioch Raghnuill.