MacMahon, Hugh (1660–1737), catholic archbishop of Armagh, was one among at least twelve children of Colla Dubh MacMahon of Cavany, Co. Monaghan, and his wife Eileen O'Reilly. He was fostered to the MacAlivery family when about seven years old. MacMahon entered the Irish College in Rome in mid to late 1683, being ordained a priest in October 1688. He probably graduated DD in 1689. He subsequently journeyed to Flanders, remaining there for seven years. In 1695 he was unsuccessfully nominated for the presidency of the Irish College, Louvain. He became (1696) canon of the collegiate church of St Peter at Cassel, where an uncle, Arthur Augustus MacMahon, held the office of praepositus to the chapter. He remained there for eleven years, despite being appointed (c.1701) vicar general of Clogher.
In April 1706 the chapter of Cassel informed Rome of their belief that MacMahon was worthy of advancement. That same year he petitioned the pope to be allowed to return to Clogher, while retaining the profits of his canonry. This was granted to him for a three-year term in September, though he did not immediately travel to Ireland. He was appointed bishop of Clogher by Propaganda Fide on 22 March 1707, being consecrated at Saint-Omer on 8 November. This appointment was noteworthy, as the Stuart pretender, James III, who claimed the right to nominate the Irish catholic episcopacy, did not nominate him, unlike six other Irish bishops appointed in 1707. Following his appointment as bishop, MacMahon again petitioned to be allowed to retain the profits of his canonry. They were conceded to him in August 1708. He immediately left Cassel, arriving in Dublin in October.
On his return MacMahon initially stayed in Dublin, only departing for Clogher sometime after May 1709. Once there, he quickly became active, attracting the attention of the priest-hunter Edward Tyrrell (qv) over the following years. In October 1712 Tyrrell had the house of MacMahon's father searched. These years also saw MacMahon enter into frequent correspondence with Rome, in which he reported on such topics as anti-catholic legislation, the state of catholicism in Ulster, and the need to appoint an archbishop to Armagh. This correspondence saw MacMahon consulted by Rome in 1710 on a dispute between two rival claimants to the post of vicar general to Armagh. Partly on his advice, one of the claimants, John Verdon, was appointed bishop of Ferns, leaving his opponent, Patrick Dowdall, as vicar general. On 22 August 1711 another clerical dispute led to MacMahon's being appointed administrator of Kilmore, a post he held until 1728, when he asked to be relieved, citing his old age.
By September 1712 MacMahon had returned to Cassel to act as executor of his deceased uncle's will. While travelling across England en route to Cassel, he was arrested and brought before a magistrate but escaped and completed his journey. In early 1714, at the behest of Propaganda Fide, he wrote a detailed report on his diocese, providing great detail on difficulties created by the penal laws for catholics. He then returned to Ireland in July 1714.
Meanwhile (16 August 1713) Propaganda Fide had decided to appoint MacMahon as archbishop of Armagh but kept this decision secret until James III's approval was obtained. On 24 May 1715 the Old Pretender grudgingly nominated MacMahon to the see, and he was formally translated from Clogher to Armagh on 5 July. For his support, he was allowed to retain the canonry of Cassel, and he also received a pension from France. His initial months as archbishop were troubled by having to avoid attempts to capture him throughout late 1715. These died down by 1716, however. Conscious of previous disputes between the archbishoprics of Dublin and Armagh over primacy, MacMahon during these years also actively sought recognition from Rome as primate of all Ireland, though this was not forthcoming.
This dispute between the two archbishoprics flared up again in 1719, as a result of the Rivers case. Valentine Rivers, incumbent of the parish of St James and St Catherine, appealed to MacMahon after Archbishop Edmund Byrne (qv) of Dublin sought to remove him. MacMahon offered to arbitrate in the dispute, but Byrne refused to accept this. Later, when Byrne refused to be represented at MacMahon's ecclesiastical court, the latter passed judgement for the appellant and sent his own clergy into Dublin to assist Rivers. In late 1719 the case was referred to Rome, which led MacMahon to write a report supporting Armagh's claim for primacy. The quarrel between Rivers and Byrne was settled in 1723, and the latter's death (1724) put the primatial dispute on hold. MacMahon, nonetheless, wrote Jus primatiale Armacanum that year to support Armagh's primacy claim. He delayed publishing it in order to reply to another piece by a Fr Hennessy, which supported Dublin's claim to primacy. This work, Prosecutio ejusdem argumenti pro Primatu Armacano contra anonymum, was completed in 1728, and both of MacMahon's compositions were published that year. They were the last major contributions to that question.
Aside from this, MacMahon busied himself in other affairs. He helped found (1722) a Dominican convent in Drogheda, to which he gave the head of Oliver Plunkett (qv). The report of 1731 into the state of popery in Armagh diocese also indicates that he busied himself establishing schools, chapels, and friaries, and providing an educated clergy for his cure. He himself took up residence in Drogheda (1728). Afterwards, he was active in ensuring clerical discipline. In 1729 Propaganda Fide appointed him to a commission to investigate accusations of malpractice against Bishop Flynn of Ardagh, who was subsequently interdicted on these charges. MacMahon carried out similar investigations on Propaganda Fide's behalf throughout the early 1730s.
By the time of his death MacMahon's actions had won him acclaim, and Pádraig Mac a Liondáin (Mac Giolla Fhiondáin) (qv) wrote poetry in his honour. He died 2 August 1737, aged 77, and was buried in St Peter's church, Drogheda. His nephew Bernard MacMahon (qv) (d. 1747), bishop of Clogher, succeeded him as archbishop of Armagh.