O'More (Ó Mórdha), Uaithne (c.1575–1600), last great lord of Leix (Laoighis), was one of seven children of Rory (Ruaidhrí Óg) O'More (qv), lord of Leix, and his wife Margaret (Maighréag), daughter of Aodh O'Byrne (qv). While nothing substantive is known of Uaithne's youth, he does not seem to have been in Leix during the English offensive of November 1577, when Robert Harpoole (d. 1594), sheriff of Carlow, and Sir Francis Cosby (qv), seneschal of Leix, launched a night assault on Rory's residence in the Gallen forests. Their mission was to rescue Sir Henry Harrington (qv) and Alexander Cosby (d. 1596) from Rory's prison. During the ensuing slaughter, two of Uaithne's brothers were killed; his mother was beheaded and her head was later paraded by the English army throughout the midlands. Although Rory was badly wounded, he escaped into the forests and onwards to his brother-in-law Fiach O'Byrne (qv). After Rory recovered, he launched devastating raids throughout Kildare and Carlow, but he was eventually cornered and killed by the MacGillapatricks of Upper Ossory on 30 June 1578, leaving Uaithne and his siblings orphans. Then Uaithne's maternal uncle, Fiach O'Byrne, stepped in and undertook to raise and educate the children in his Wicklow heartland of Glenmalure. However, Uaithne's first years in Glenmalure were spent surrounded by conflict because of Fiach's determination to oppose the extension of English power and protestantism into Leinster and Ireland. In 1582 Uaithne and his siblings suffered another blow when two half-brothers, with some O'Tooles, were publicly executed at Dublin for their role in the Leinster war of 1580–83.
Fiach took a great liking to the young Uaithne, becoming his mentor – schooling him in politics and warfare. It is clear, though, that Fiach was waiting for the day when Uaithne would return to Leix and assume his late father's lordship. That time came in August 1594, when Queen Elizabeth reintroduced martial law (15 August) throughout Leinster to curb the activities of Fiach. The reaction of the lord of Glenmalure was immediate, dispatching Uaithne to raise hell among the midland planters to divert English forces from the Wicklow theatre. As Uaithne was barely out of his teens, Fiach sent the seasoned Piers Grace (fl. 1596) to advise the young man. Soon the pair had Leix burning. Fiach was forced to come to terms in June 1595, ordering Uaithne to observe a ceasefire. They, however, were biding their time. In early 1596 Fiach decided to return to war. Accordingly he authorised Uaithne, Grace, the Kavanaghs, the O'Farrells, and his Butler allies to burn throughout the midlands that spring. At the same time, he remained outwardly loyal but orchestrated events like a puppet-master. In Leix Uaithne immediately teamed up with some MacGillapatricks, exploiting the relatively weak position of the planters of Leix. At Stradbally Bridge on 19 May, he settled scores with the Cosbys, routing them. According to Sir Thomas Colclough (d. 1624) of Tintern, Co. Wexford, on 21 June Uaithne and Fiach were assembling troops in the forests north of Mount Leinster. Colclough wrote that Phelim O'Byrne (qv), on behalf of Fiach, had held a major conference there with Gerald Kavanagh (d. 1596) of Garryhill and Uaithne. He again highlighted the emerging danger in east Leinster, reporting that Uaithne crossed the Barrow at Carlow early on 25 June to rendezvous with the Butlers and the Kavanaghs. According to Colclough, Piers Butler (d. 1597) at this meeting swore oaths binding himself to Domhnall MacMurrough Kavanagh (qv), king of Leinster. After that they and Uaithne travelled on to an evening conference with Fiach in the Coolatin forests of south Wicklow.
These developments were profoundly worrying for the English administration in Dublin and for the planters of the midlands. Any lingering doubts of a large catholic military build-up in Leinster were dispelled when Uaithne again appeared in Wicklow to take instruction in early July. Significantly, the messengers of Hugh O'Neill (qv), 2nd earl of Tyrone, were also in Fiach's camp at this time. Quite clearly Uaithne was at the centre of the conspiracy of Fiach and Tyrone with the Spanish against the English government. Central to the plans of Fiach and Uaithne was their determination to draw the nephews of Thomas Butler (qv), 10th earl of Ormond, irreversibly into the conspiracy. On 3 August 1596 the alliance with the Butlers was sealed in a solemn covenant on Barnacashel Hill, Shillelagh. This commitment was doubly sealed later that month by the marriage of James Butler (d. 1597) to Doireann, sister of Uaithne. On 13 August the commander of Ballinacor fort in Glenmalure urgently dispatched a letter to the lord deputy, William Russell (qv), stating that Uaithne, Butler, and the Kavanaghs had arrived at Fiach's camp a day earlier. Indeed, Uaithne's presence with Fiach was an act of brilliant manipulation. On 7 August he agreed to a truce of eight days with English forces in Leix, which freed him for the Wicklow campaign. Correspondence also mentions that Butler hanged six members of the Ballinacor garrison after granting them quarter, before attacking the fort. Apparently, Uaithne and Fiach had made the hangings a precondition for the Butlers to gain access to the catholic inner circle of command. After another attack on 16 August, Ballinacor eventually fell on 9 September. This signalled the outbreak of widespread violence throughout Leinster in preparation for the landing of a Spanish army. But the catholic leaders of Leinster were dealt a crushing setback when the Spanish fleet was wrecked by storms on 13 October. And after fierce fighting throughout October and November, the war began to go badly for them. Fighting continued into 1597. In January Tyrone now moved a step closer to outright war with the English, sending Capt. Richard Tyrrell (qv) into the midlands, where he linked up with Uaithne, but was wounded in a skirmish. In spite of this, Uaithne ensured that Tyrrell was able to meet Piers Butler and Fiach's messengers. But Fiach's killing on 7 May 1597 was a huge setback to the Leinster catholics.
The untenable military situation in Leinster perhaps spurred Tyrone to encourage Domhnall MacMurrough Kavanagh and the sons of Fiach to withdraw into the safety of Ulster. They were joined on their trip northward by Uaithne and his elder half-brother, Brian Riabhach O'More (qv). It was only in October 1597 that Tyrone considered it safe enough in Leinster for them to return to continue the war. On 18 October it was reported that they, with 800 troops under Brian Riabhach, were burning towns within six miles of Dublin before attacking the English of Wexford. While Brian Riabhach continued to attack the English in East Leinster, Uaithne and his brother Edmund O'More (fl. 1632?) were dispatched to Leix to attack the plantations. There they won a major engagement on 7 December against Warham St Leger (qv) (d. 1600), governor of Leix. Despite Tyrone's truce of December 1597 with the English, fighting continued in the midlands, although it seems to have largely ceased in east Leinster. Sometime in early 1598 Uaithne travelled northward to Ulster to confer with Tyrone and ask him for military assistance. The death of Brian Riabhach, from wounds received while fighting Ormond, left Uaithne as the principal catholic commander in the midlands. In August Tyrone ordered Uaithne, Remainn (Redmond) Burke, and Tyrrell to extend the war into Munster. Before leaving for Munster, Uaithne appointed his brother Edmund as his lieutenant in Leix. During the Munster campaign, Uaithne and his allies devastated the province from north Tipperary to Kilmallock, Co. Limerick, installing James fitz Thomas Fitzgerald (qv) (d. 1607) as earl of Desmond before returning to Leinster.
Central to Uaithne's strategy in the midlands was his determination to take the fort of Maryborough. This he attempted to do through direct assaults and by ambushing its supply trains, such as his mauling of Ormond's relief column at Blackford in late 1598. In 1599 the whole of Leinster was again engulfed by warfare. Uaithne thrived in these circumstances, enjoying spectacular success. On 17 May he with Tyrell inflicted an embarrassing reverse on the rearguard of Robert Devereux (qv), earl of Essex and lord deputy of Ireland, in an ambush known as the battle of the Pass of the Plumes. Twelve days later, Uaithne was also part of Phelim O'Byrne's victory in Wicklow, the annihilation of the column of Sir Henry Harrington. The effect of these successes was blunted somewhat by Essex's mauling of the Leinster catholics on 29 June outside Arklow; but in spite of this, catholic resistance in Leinster continued in 1600. On 16 April Uaithne earned a fortuitous and unexpected success, capturing Ormond at Corrandhu near Ballyragget, Co. Kilkenny. Then Uaithne, with the Jesuit Fr James Archer (qv) (d. 1620), attended a parley with the earl and several English officials. During the proceedings Ormond became abusive and threatening to Archer. Uaithne's supporters, fearing that Ormond was about to draw his sword on the priest, apprehended the earl and drove off his troops. A somewhat surprised Uaithne then spirited Ormond into Leix before lodging him in the MacGillapatrick castle of Gortnaclea. There Archer unsuccessfully tried for three weeks to convince the earl to return to the catholic faith of his ancestors and to join Tyrone. Either at the end of April or the beginning of May, Uaithne transferred him back into Leix and onwards to the O'Dempsey fortress of Ballybrittas. It was reported that Tyrone wrote to Uaithne, asking him to deliver Ormond to him. Uaithne refused, indicating a considerable independence in mind and in his military command. Ormond remained in Ballybrittas castle until terms were arranged on 12 June for his release.
Despite this success, 1600 was the decisive year of the war in Leinster. Essex was replaced as lord deputy by Charles Blount (qv), Lord Mountjoy. In comparison with the somewhat erratic Essex, Mountjoy was systematic in his approach to the war in Leinster, which led to heavy fighting throughout the summer. Secondly, because of the dramatic increase in pressure on the Leinster catholics, Uaithne, with his brother Fiach the most talented catholic commander, died between 11 and 17 August 1600 of gunshot wounds received in battle with Mountjoy's forces near Vicarstown. His loss was a disaster, undermining the catholic resolve to fight, as shown by the submission of Domhnall MacMurrough to Mountjoy on 24 August 1600 near Cashel; although Uaithne's brother Remainn continued to fight on for a while before he eventually surrendered in 1601, effectively ending resistance in the midlands. Uaithne seems to have married, but to have died childless.