Thomas (d. p.1213), abbot of Glendalough, belonged to the Uí Muiredaig dynasty of north Leinster. In the Latin Life of Lorcán Ua Tuathail (qv), he is described as a nephew of the latter, which probably means that he, too, belonged to the family line of Ua Tuathail. Thomas's parents are not identified; though he granted land near Newcastle, Co. Wicklow, to the abbey of St Thomas, Dublin, ‘for the benefit of the souls of his father and mother’, he does not name either of them. However, as Thomas was presumably born by c.1140 at the latest, it seems likely that his father was Lorcán's eldest brother, Augaire Rua. In turn, Thomas was apparently married, although there is no record of his wife. He had at least one son, Alexander ‘son of the abbot of Glendalough’ who, towards the close of the twelfth century, witnessed a grant of land to St Mary's Abbey, Dublin. His personal name, like that of Thomas (Tomás?) himself, is rare, but not unattested, in pre-Norman Irish genealogies. Record of a grandson, Richard, son of Alexander, who witnessed the surrender of Balicogan to the abbey of St Thomas, Dublin, at some date in the early thirteenth century, may point to some Anglo-Norman influence.
Thomas's predecessor as abbot of Glendalough, whose name is given in a corrupt form as ‘Edenigmus’, is among the witnesses to the foundation charter of the priory of All Hallows, Dublin, c.1162 or 1163, after the overking of Leinster, Diarmait Mac Murchada (qv), had strengthened his hold on Dublin and Lorcán had been promoted to the archbishopric. Thomas, although aged only in his early to mid twenties at the time of his uncle's promotion, was (according to the Life of Lorcán) deprived of his rightful succession to the abbacy on this occasion. Despite his uncle's close working relationship with Mac Murchada, it is claimed that the latter imposed as abbot an unnamed cleric, almost certainly to be identified with ‘Edenigmus’, which is difficult to explain except in terms of political expediency whereby the king opted to appease a rival party (possibly an Uí Máil lineage) at the ecclesiastical centre. It seems clear that Thomas felt aggrieved with Mac Murchada, all the more so in later years when the latter's erstwhile Anglo-Norman allies had taken control in Leinster and were threatening the very survival of Glendalough.
Meanwhile, Thomas managed to displace his rival – either in 1163, when several churches at Glendalough (including Cró Cóemgin, Cró Ciaráin, and the church of the two Sinchells) were burned, which may indicate disturbance, or, perhaps more likely, in 1166–7 during Mac Murchada's enforced exile from Leinster. The Life of Lorcán stresses that the choice of Thomas as abbot by the clergy and tenants of Glendalough was because of his merits and not his lineage, suggesting a perceived need to justify his tenure of office. The same source emphasises his devout character by relating how, when travelling with Lorcán and two other bishops, his prayers helped to heal a possessed woman. Whatever the case for Thomas's religious virtues, it is hard to reconcile his abbatial appointment, as a married representative of a Glendalough-linked dynasty, with his uncle's presumed commitment to church reform; the reforming party were as insistent upon clerical celibacy as they were on ending lay headship of churches. The appointment of Thomas, presumably with the support of unreformed clergy and dynastic elements at Glendalough, illustrates the extent to which reformers like his uncle, Lorcán, were obliged to compromise in order to progress.
It is not clear if Thomas's position as abbot was threatened, in the immediate term, by the death of Mac Murchada (1171) and the seizure of power in Leinster by the English. Sometime after 1172 (perhaps in 1173, when Cináed Ua Rónáin, bishop of Glendalough, died), he secured confirmation from Richard de Clare (qv) (Strongbow) who was by this time married to his cousin Aífe (qv), of an earlier Mac Murchada grant of the abbatial properties of Glendalough. The fact that witnesses to the charter included not only Aífe but Lorcán suggests that, whatever differences they may have had in regard to church reform, uncle and nephew had a good working relationship. During these same years Thomas facilitated Lorcán by witnessing grants of land to the cathedral of the Holy Trinity. On his own behalf, he made the already-mentioned grant to the abbey of St Thomas, and also ceded property to St Mary's Abbey.
Evidently, Thomas's situation deteriorated following the death of Strongbow in 1176, and the actions of Robert le Poer (qv), which led to the expropriation of the Uí Muiredaig kingdom in 1178. It seems that these developments formed the context for land-grants to Glendalough by his uncle, Lorcán, perhaps to provide sanctuary for displaced Ua Tuathail dynasts on ecclesiastical properties. Thomas came under increasing pressure after the death of Lorcán in 1180, and the appointment to the archbishopric of Dublin of the English prelate John Cumin (qv). However, he proved to be a strong defender of the rights and possessions of the abbacy. In 1185/6, and again in 1192, when John (qv), lord of Ireland, made grants (which proved to be ineffective) of the bishopric of Glendalough to Cumin, he secured confirmation of his abbatial office and properties from King Henry II (qv) and from John himself.
A further challenge for Thomas was the appointment, from 1192, of the Anglo-Norman William Piro as bishop of Glendalough. The latter, it seems, did obtain possession of his see and was probably responsible for the reconstruction of the cathedral, using Dundry limestone, towards the close of the twelfth century. Similarly, it is reasonable to attribute to Thomas the construction, opposite the cathedral, of a small ecclesiastical building, known as ‘the priest's house’, also datable to the end of the twelfth century and possibly intended as a shrine for the relics of the founder saint of Glendalough, Cóemgen (qv). Over the door a tympanum, now broken, illustrated a bishop and a cowled figure attending an enthroned ecclesiastic; this was perhaps a statement on Thomas's part to the effect that the bishop, and the Augustinian prior alike, owed obedience to him as the true successor of St Cóemgen.
It is clear that Glendalough, in the later years of Thomas's abbacy, presumably because of the sheltering of dispossessed Leinster dynasts on ecclesiastical lands, was viewed by the English administration in Ireland as a hotbed of political dissent – which in turn accelerated efforts to unite the diocese with Dublin and extinguish the abbacy. When papal approval of Dublin's claims was ultimately secured in 1216, testimony in support of the union included assertions that Glendalough was ‘desolate for almost forty years’, and was ‘a den of thieves and a pit of robbers’, claims which fit ill with what is known of building projects, of cultural development, and of education at the ecclesiastical centre in Thomas's time. During these years, Thomas supported the cause, initiated by the canons of Eu, for the canonisation of his uncle, Lorcán, and facilitated the anonymous author of the ‘Vita S. Laurencii’ by supplying the Glendalough background for the Life. This, presumably, explains the positive bias of that source regarding his claims to the abbacy. Meanwhile, he continued to pursue confirmation of his abbatial possessions, obtaining a bull of protection from Pope Innocent III on 22 December 1198. He secured further confirmations from John, after the latter became king. On 30 October 1200 King John conceded the possessions of the abbacy to Thomas for life, provided they did not exceed 40 carucates, although this stipulation is difficult to reconcile with the list of properties given in the bull of only two years earlier. Sustained pressure from the English administration on Thomas's position continued until the end of his days. The succession to the archbishopric of Henry (qv) of London on the death of Cumin (1212), and the vacancy in the bishopric of Glendalough following the death of Piro, furnish the context for a charter of King John, dated 30 July 1213, which specified that the possessions of the abbacy, reserved to Thomas for life, be transferred to Archbishop Henry after the abbot's death or retirement. This, the last historical reference to Thomas, which clearly implies that his departure from office was viewed as imminent, has prompted assumptions that he was the last abbot of Glendalough.
However, it appears that Thomas, who died sometime after this, aged presumably in his seventies, had at least one successor. The register of Archbishop Alen preserves mention of Tadc Ua Tuathail (‘Tadeus Otothyll’), clearly an Uí Muiredaig cleric, styled ‘abbot of Glendalough’, who granted the vill of Cill Moccu Birn (townland of Killickabawn, parish of Kilcoole) and the church of Dísert (Desertkevin, near Glendalough) to Archbishop Luke, at some date between 1228 and 1255. Record of the abbacy ends with Tadc. Thereafter, such testimony as survives alludes to priors of the Great Church of Glendalough, who were subject to the prior of the Holy Trinity, Dublin.