O'Carroll (Ó Cearbhaill), Tadhg Ailbhe (d. 1407), king of Éile, was son of Tadhg son of Ruaidhrí O'Carroll, king of Éile (Ely O'Carroll), and an unknown woman. He was probably the most powerful and well-known king of Éile in the middle ages. Nothing is known of his childhood except that he was left fatherless at a very early age. His father Tadhg O'Carroll (d. 1346), son of Ruaidhrí, enjoyed a short life, according to the annals that describe him as a ‘young prince’ of Éile. However, in life he proved himself an accomplished warrior and the reconqueror of the O'Carroll patrimony. Indeed, he continued the policies, dating from the late 1310s, of his immediate predecessor, Donnchadh Fionn O'Carroll (fl. 1317). In 1345 Tadhg son of Ruaidhrí, with Diarmait MacGillapatrick and Conall O'More (qv) (d. 1348), joined the 1st earl of Desmond's (qv) rampage through the Butler lands in Ely and Ormond. But their failure to take Nenagh, and the expedition of the justiciar Ralph de Ufford (qv) in September and October that year, put them at a disadvantage. However, Tadhg son of Ruaidhrí and his MacGillapatrick allies returned to expose the fragility of the English position in the midlands, burning Bordwell in the cantred of Aghaboe in December. This state of serious war continued into 1346. In what seems to have been a coordinated campaign, the O'Mores, O'Connors of Offaly, MacGillapatricks, O'Carrolls, and O'Dempseys confederated themselves to wreak havoc on the English in the midlands. In Easter 1346 Conall O'More, the O'Connors of Offaly, and the O'Dempseys captured the FitzGerald castle of Lea and destroyed several other castles. Their actions are arguably linked to the burning by Tadhg son of Ruaidhrí and his MacGillapatrick allies of Aghaboe manor that year. Even though Tadhg son of Ruaidhrí was killed in battle during September 1346 against the English of Ossory, he succeeded in expelling the Brets, Milbournes, and other English from Éile and occupying their lands. It would seem that Tadhg son of Ruaidhrí was succeeded as king of Éile by his kinsman Ruaidhrí O'Carroll (fl. 1361), who continued Tadhg's aggression against the English territories close to Éile. Clearly he took part in the great disturbances orchestrated by Domhnall O'Kennedy and Brian Bán O'Brien (qv) that raged throughout the midlands in 1347–8. Both the O'Carrolls and Brian Bán chose to fight on after the hanging of O'Kennedy at Thurles in spring 1348, leading to major campaigns against them during that summer. While the English may have enjoyed a temporary victory, Ruaidhrí remained obdurate and truculent in his desire to continue the conflict, and in 1349 he and the MacGillapatricks devastated much of northern Tipperary and Kilkenny before finally capturing the castle at Aghaboe. Its fall, despite the 1351 expedition of the justiciar, Thomas Rokeby (qv), ensured that the cantred of the same name was effectively lost to the English of Ossory. The virtual collapse of the midland colony ushered in a long period of peace among the O'Carrolls. Ruaidhrí was in turn succeeded by William Alainn O'Carroll (d. 1375).
Tadhg Ailbhe succeeded to the kingship on William's death. Even at this stage Tadhg was a major figure in the political world of the midlands. This was borne out by the fact that his first wife was Johanna (d. 1383), daughter of James Butler (qv) (d. 1382), 2nd earl of Ormond. They married probably sometime in the early 1370s, as by the late 1370s Tadhg was virulently opposed to the Butler earls of Ormond. This hostility manifested itself in his first real action of note, during June 1377, when he and Murchadh na Ráithníghe O'Brien (qv) of Arra brought their forces from the midlands into Leinster to join with Art MacMurrough (qv), king of that province, with whom they ravaged much of Kilkenny, Carlow, and Kildare, enforcing the Leinster king's developing hegemony over the strategic Barrow valley. Any remaining vestige of Tadhg's former alliance with the Butlers died with the death of his wife in 1383. Shortly afterwards Tadhg married Mór, daughter of King Brian Sreamach O'Brien (qv) of Thomond. For the rest of the 1380s Tadhg remained at peace and consolidated his rule. But in 1391–2 he answered MacMurrough's request to attack Carlow. This attack proved devastating, resulting in the sack of the town and castle. Tadhg's renewed closeness to MacMurrough during this period was in response to the threat of James Butler (qv), 3rd earl of Ormond. Ormond and his brothers were determined to reassert Butler overlordship over the Irish of Ossory and the O'Carrolls, endangering Tadhg's relative autonomy. For Tadhg, the defeat of MacMurrough by Richard II (qv) in October 1394 was a considerable setback, but he did accept the agreement concluded by MacMurrough with Richard. Characteristically, Tadhg tried to turn this situation to his advantage: in 1395 he wrote to the king, requesting that he be recognised as the king's immediate subject and asking for aid against his dynastic rivals. In particular, he was desirous of royal protection from the designs of Ormond. However, this acceptance involved his eating humble pie. Accordingly, on 25 April 1395 Tadhg, along with Toirdhealbhach O'Brien (qv) of Arra (d. 1399), the two O'Dwyers, and three O'Kennedy leaders, submitted as Butler clients at Kilkenny. Ormond, however, intended to enforce his suzerainty more forcefully over Tadhg and these Irish lords once Richard departed. Indeed, Tadhg and the O'Connors of Offaly suffered a series of English attacks throughout summer 1395 in contravention of the agreement. In spite of such provocation, the agreement held. Tadhg took full opportunity of this lull, embarking on a pilgrimage to Rome in early 1396. There he saw the ruins of ancient Rome and displayed his devotion at several centres of pilgrimage. On his way home, he met Donnchadh O'Byrne (qv) at Richard's court in London. Together they joined the king's campaign in August 1396 to Calais and performed good service in France.
It seems that Tadhg eventually came home to Éile either in late 1396 or early 1397. He returned to a country in crisis as the agreement steadily imploded. While Richard's second campaign to Ireland in summer 1399 does not seem to have affected the O'Carroll territories, Tadhg, because of Ormond's aggression, clearly felt a considerable affinity for MacMurrough. After the humiliation of Richard, MacMurrough resolved to attack Ormond, travelling to Munster in August 1399 to aid the Desmonds against the Butlers. He then unleashed Tadhg on the northern Butler lands. But Ormond quickly neutralised the O'Carroll leader, capturing and imprisoning him till his escape from Gowran (1400). After this, Tadhg faded from the political scene, indicating that his lordship was enjoying a long period of stability. In August 1407, after defeating a branch of the O'Kennedys that were traditionally allied with the Butlers, Tadhg again answered MacMurrough's calls. Then the justiciar Stephen Scrope (qv) and James Butler (qv), 4th earl of Ormond, invaded MacMurrough's heartland of Uí Cheinnselaig. With William Burke of Clanwilliam, Tadhg set out to MacMurrough's aid. But on 9 September English forces at the pass of Callan destroyed the army of Tadhg and the Burkes as they tried to enter Leinster. In the slaughter Tadhg, the most important king of Éile in the medieval period, was killed. He left two recorded daughters, the famous Mairghréag (qv) (d. 1451) and Isabella (fl. 1445), and was succeeded by his son Maolruanaidh O'Carroll (qv) (d. 1443).