O'Byrne, Feilim (d. 1631), lord of Críoch Raghnuill, was second son of Fiach MacHugh O'Byrne (qv), lord of Críoch Raghnuill and leader of Gaelic Leinster, and Sadhbh, daughter of Domhnall mac Cathaoir Kavanagh (qv) (Mac Murrough Kavanagh). He was an able soldier and a catholic politician from the closing years of the sixteenth century to his death. In the 1590s Feilim proved an able lieutenant of his father during the latter's confederation with Hugh O'Neill (qv), 2nd earl of Tyrone, against the government. Indeed, he was present at the firing of the castle of Ardree, Co. Kildare, which led to the burning to death of Sir Piers FitzGerald and his family (18 March 1594). Evidence tends to suggest that Feilim and his brother Réamann were estranged from their elder brother Toirdhealbhach, Fiach's heir; but after the execution of Toirdhealbhach at Dublin (18 July 1595) Feilim assumed the role of his father's heir and principal lieutenant.
After Fiach's death at the hands of English soldiers (8 May 1597), Feilim succeeded as lord of Crioch Raghnuill. The weakness of his military position forced him and Réamann to flee to Tyrone during May 1597. It is clear that Tyrone was impressed by Feilim's military ability, as he appointed him commander of the Blackwater fort near Armagh. They returned briefly to Leinster in the closing months of 1597, but the prevailing military climate forced them to return to Tyrone. In January 1598 Feilim submitted and was allowed to live freely within his ancestral lands; but in November 1598 he burned Newcastle McKynegan and routed the forces of Sir Henry Harrington, seneschal of the O'Byrnes’ country and Crioch Raghnuill (28 May 1599). Despite this success, Feilim and the rest of the Leinster Irish were quickly defeated shortly afterwards by Robert Devereux (qv), lord deputy of Ireland and 2nd earl of Essex. With Tyrone's encouragement Feilim, with Uaithne O'More (qv) (sl. 1600) and Domhnall Spáinneach Mac Murrough Kavanagh (qv) (d. 1632), king of Leinster, continued the war and caused havoc throughout Leinster in 1600. However, Lord Deputy Mountjoy (qv) disturbed Feilim's Christmas festivities by his attack on Ballinacor on Christmas day 1600. While Feilim escaped through a window, his wife and son were captured. After considerable resistance, Feilim and Réamann submitted before the Dublin council (March 1601). The government was inclined to mercy, introducing a policy to pacify the formerly disaffected catholic Leinster aristocracy and to detach them from Tyrone. On 5 May Domhnall Spáinneach Kavanagh was granted his pardon, while the O'Byrne brothers and the O'Tooles of Castlekevin were pardoned in July. This decision was underlined by their refusal, despite Tyrone's urgent request, to attack government forces in Leinster after a Spanish expeditionary force under the command of Don Juan del Águila (qv) landed at Kinsale in September 1601.
No doubt Feilim was anxious to avoid the fate of his uncle by marriage, Feilim mac Toirdhealbhach O'Toole of Powerscourt, whose murder by the Wingfields (14 May 1603) paved the way for them to receive a grant of the dead man's estates. As a result Feilim embarked upon the transition from warlord to nobleman, declaring his loyalty to James I. On 4 April 1604 Feilim was granted certain lands by letters patent from James I, but in October 1605 he bitterly quarrelled with Sir Arthur Chichester (qv), the lord deputy, refusing to accept his patent unless the whole region was granted to him. His demands were staunchly resisted by the Irish freeholders, including his brother Réamann. On 16 January 1606 an inquisition deemed that Fiach had died in rebellion and his lands were forfeit, but his personal estate was divided between Feilim and Réamann, not including those lands gained by mortgage or conquest. After protracted negotiation, the brothers agreed a solution and Wicklow was shired on 6 March 1606. Although now part of the new order, they remained in contact with Tyrone in continental Europe. And on 4 June 1610 Tyrone's messenger was captured bearing letters to Feilim and Réamann.
Feilim served as a justice of the peace and as a member of parliament for Wicklow, and his name appears among the signatories of a petition sent to James I by catholic members of the Irish parliament in 1613. However, on 1 February 1616 his lands of Cosha, between Aughrim and Shillelagh, were seized by the crown, and Sir Henry Belling, high sheriff of Wicklow, delivered them to Sir Richard Graham. On 4 November 1616 James I instructed the lord deputy, Oliver St John (qv), to regrant Cosha to Feilim and his heir, Brian. An inquiry, chaired by Sir Laurence Esmonde (qv), ruled (18 December 1617) that Feilim had no rights of inheritance to Cosha. The legal contest continued into 1620, but when St John and the Dublin council recommended to the privy council that Graham's case be accepted, Feilim had lost. Lord Deputy Falkland (qv) began another campaign against Feilim and his sons in 1624, when he imprisoned two of Feilim's sons on suspicion of being in league with Murchadh Bacach Kavanagh, who was allegedly plotting a rebellion with Spanish support against the crown. To prevent a future rebellion, the lord deputy recommended the plantation of Críoch Raghnuill, but this was rejected by the commissioners for Irish affairs.
In 1625 Feilim, as justice of the peace, apprehended Brian Kavanagh, brother of Murchadh Bacach, with two others, and hanged them for the murder of a land speculator, the Rev. Robert Ponte of Rathdrum (also a justice of the peace). However, it was suspected that Feilim authorised the murder, and Ponte's daughter Margaret later alleged that Aodh O'Byrne (qv), Feilim's second son, was the actual murderer. The controversy gave Feilim's enemies their chance. His brother Reámann and Cathaoir O'Byrne, with Sir Laurence Esmonde, formulated a plan to accuse Feilim and his sons of the murder. A commission was established to investigate Feilim's title to Críoch Raghnuill, and O'Byrne and his sons were tried for murder. In August 1628 Feilim and his sons were convicted of Ponte's murder and imprisoned in Dublin castle. On hearing the verdict, Feilim's wife, Úna, suffered a heart attack and died within a few days. However, through the effective lobbying of his son Hugh and son-in-law John Wolverston, and the help of powerful friends at court, Feilim's plight was recognised. In December 1628 the commissioner for Irish affairs, supported by the privy council, confirmed Feilim's lands, but increased his rents payable to the crown. Feilim and his sons were released and allowed to return to Críoch Raghnuill, but this did not prevent an influx of planters into his lordship.
The subsequent bitterness engendered by the plantation of Críoch Raghnuill erupted during the rebellion of November 1641. Feilim died between 1 and 24 January 1631. He married (a.1593) Úna, daughter of Fiach O'Toole (qv) (d. 1578) of Castlekevin, and sister of his stepmother, Rose O'Toole (qv); they had probably nine sons and four daughters. He was succeeded by his eldest son, Brian O'Byrne (qv) (d. c.1632).