Ó Briain, Mairín (Marianus, Maurianus) (d. 1238), archbishop of Cashel, belonged to an offshoot of the north Munster dynasty of Dál Cais. Although his parentage is not recorded, he was presumably related to the king of Thomond, Donnchad Cairprech O'Brien (qv) (d. 1242), son of Domnall Mór Ua Briain (qv). In all probability, he belonged to an ecclesiastical line within the dynasty. Whether he carried his forename from birth, or adopted it on entering the religious life, it appears that he did have a strong Marian devotion; his personal seal included a representation of the Blessed Virgin. An Augustinian canon, he was elected to the bishopric of Cork in 1215 against a protégé of King John (qv), although he appears to have gained the latter's confidence. He served in this capacity till 1224, when he was promoted to the archbishopric of Cashel. In his new role, he became a strong defender of the rights of his see, and a bulwark against the extension of English control over the Irish episcopate. At the same time, the indications are that he was not racist: he worked with English officials and functionaries alike, once he was satisfied that they were not threatening his position.
The elevation of Mairín Ó Briain to Cashel took place against a background of conflict with an English administration in Ireland which, since the death of King John (1216), had taken advantage of the minority of Henry III to extend its influence in both the secular and ecclesiastical spheres. Ó Briain's predecessor in the archiepiscopate, Donnchad Ua Longargáin (qv), who represented a Dál Cais interest, had been deprived of control over the town of Cashel by the English, who constituted a strong presence in eastern Thomond. After the resignation in 1223 of Donnchad, who had failed to re-establish his temporal power, the election of Mairín took place (19 August 1223) in dramatic circumstances. When a papal nominee, Michael Scottus, an eminent scientist, declined the see on the grounds that he did not speak the Irish language, the English administration in Ireland sought to enlarge the chapter of Cashel to secure the election of a candidate of their choice. However, Pope Honorius III, clearly informed of the racial conflict in the Irish church, refused permission to enlarge the chapter, which then elected Mairín. The pope recommended that his election be endorsed by the English crown, pointing out that the archbishop-elect had been previously accepted by the late King John.
Archbishop Mairín established his corporation spiritual in accordance with the terms of a papal bull dated 6 May 1224. Although he received royal confirmation of his temporalities on 25 August 1224, it took somewhat longer for him to realise his rights in practice. One of the first difficulties he faced was the continued presence in the diocese of Killaloe of an English nominee to the bishopric, Robert Travers. The latter had been intruded into the see in 1217 by the English administration and, although unable to maintain control of the western part of the diocese, had managed to entrench himself in the east where there was a strong English presence. A papal mandate of 9 May 1226 authorised Mairín to remove Travers – much to the relief, presumably, of Dál Cais.
By 1228 Mairín felt sufficiently confident to revive the former conflict concerning control of Cashel. Perhaps he benefited from the fact that King Henry III around this time took charge of affairs of state; control of the town was conceded and, two years later, the archbishop issued his own charter, providing for a corporation temporal with a provost and twelve burgesses. In the years that followed, he oversaw the development of his see. His seneschal, Sir David Latimer, whose name indicates his English origin, played a key role in the establishment of a leper hospital at Cashel, dedicated to St Nicholas. The archbishop also initiated the new Gothic cathedral on the Rock, although the development of the building was directed by his successors over a span of more than half a century.
In 1231 Mairín travelled to Rome but took ill on the way and, as an act of penitence, adopted the Cistercian habit. He recovered, however, and returned to Ireland. The closing years of his archiepiscopate were overshadowed by renewed conflict with the English administration. Some time before January 1235 he wrote to the pope (Gregory IX), seeking a mandate to initiate a report on the ‘intolerable’ customs in Ireland. This, it seems, was not acted on.
Mairín Ó Briain resigned around 6 June 1237, and died the following year 1238 . He is buried in the Cistercian house of Inislounaght (near Clonmel), which had been re-established some sixty years earlier by the powerful Dál Cais ruler, Domnall Mór. His immediate successor at Cashel was a Dominican preacher, Dáibid ‘mac Ceallaigh’ (David O'Kelly), son of Ceallach Ó Giollapátraic, who was promoted from Cloyne.