MacMahon (McMahon), Michael Peter (c.1710–1807), catholic bishop, was probably born in Doonass, Co. Clare, son of Patrick MacMahon of Dooradoyle, Limerick, and Margaret MacMahon (née O'Sullivan). From a wealthy catholic gentry family who lost their lands in the Williamite confiscations, MacMahon joined the Dominicans, studied in Lisbon and received tonsure, minor orders, and subdiaconate there (1736). Returning to Ireland, he was appointed to the Dominican house in Limerick, served as vicar provincial and principal examiner of the province, and was prior as of 1749. Considered for the bishopic of Cork (1763), he was coadjutor of Killaloe (1764) before succeeding William O'Meara as bishop of Killaloe (1765–1807). While his theological training and twenty years on the Irish mission were important factors, his selection owed much to his good birth and the financial security it afforded. Throughout his episcopacy, the bishop received annuities from his brothers on the Continent. Maurice (d. 1791) was a member of the Irish brigade in France, fought at Culloden (16 April 1746), and was made a knight of Malta. John Baptist was a doctor in Autun, Burgundy, and later became, by marriage, the marquis d'Éguilly. Through him, one of the bishop's grand-nephews was Edmé Patrice Maurice MacMahon, president of the French republic 1873–9.
MacMahon was consecrated at Thurles (5 June 1765) by James Butler (qv), archbishop of Cashel, and on the same day was granted a mastership, an honorary degree of the Dominican order. His long episcopacy was personally and politically eventful. Early on, he complained to Rome (1769) of the financial difficulties priests and parishioners faced under the penal laws. He noted the decline of regular clergy and was critical, too, of Irish students who, after training in France, failed to complete their Irish mission. MacMahon was also the next-to-last Irish bishop to be nominated by the Old Pretender, James III. After James's death (1 January 1766), Rome refused to give recognition to his successor Charles Edward. In Ireland, the papal denial of Stuart claims strengthened the chances, often proposed but never achieved, of formal recognition of catholic clergy in exchange for an oath of allegiance. Opposition was weakened by the first signs of catholic relief and the death of the bishop's friend and fellow Dominican Thomas Burke (qv), bishop of Ossory, a strong opponent of such an oath. Archbishop Butler took the lead (1775), and after the passage of the initial relief acts MacMahon himself took the oath (1778).
Already in his seventies, MacMahon and his future coadjutor, James O'Shaughnessy, narrowly escaped a fire in Mogaullane, Newmarket-on-Fergus, by running naked into the night (1785). More generally, as agrarian discontent in the 1780s began to include hostility to the fees charged by catholic clergy, the bishop and other Munster clergy found themselves on the defensive. When, for example, he tried to dissuade the parishioners of Castleconnell (July 1786) from joining the Rightboys, they walked out of the church. The bishop also personally supported a school at Killaloe (c.1767–1792) and, in the détente of the 1790s, erected a schoolhouse on ground adjoining the Ennis chapel (1792). He began a subscription for the school, a large amount being donated by protestants. ‘I mention these’, he wrote in a report to Rome, ‘knowing that it will be agreeable to you to hear what liberal sentiments at present subsist betwixt Roman Catholics and protestants’ (Fenning, ‘Two diocesan reports’, 27). In that spirit, MacMahon was one of five catholics granted the freedom of Limerick as a gesture of goodwill (February 1794). From 1795 to 1800, the bishop was involved in an ongoing dispute with Henry MacDonogh, priest of Castleconnel and Athane. When MacMahon suspended MacDonagh for repeatedly ignoring his orders, the priest briefly detained and verbally abused the octogenarian bishop. Rival factions developed. The priest refused to leave the parish, church construction there was halted, and repeated attempts to mediate the dispute failed. The crisis seems to have ended only when Maurice McCormick, parish priest of Killaloe, resigned his position to accommodate MacDonagh's transfer outside the diocese.
In the debates on the act of union of Britain and Ireland, MacMahon signed a resolution in support of the union, in the understanding that catholic emancipation would follow (September 1799). Having reached the age of 97, his life extending almost the whole of the penal era, the bishop died at midnight on 20 February 1807 in his winter residence of Lock Quay in Limerick (he had another at Cappavilla House (Cappaville) near Limerick in the parish of Kiltenanlea in the diocese of Killaloe). His funeral procession included not only catholic clergy, but the mayor and a number of protestant gentlemen. Though subsequent demolition has effaced its precise location, he is buried in the Church of Ireland's St John's church or churchyard, Limerick, in the same vault as the Rev. John Thayer (1758–1815), a noted catholic controversialist, and his relative William Hartney. There is a portrait of the bishop in St Flannan's College in Ennis, Co. Clare, and there remain in use in the diocese two gold pectoral crosses handed down from MacMahon to his successors.