Ó Longáin, Mícheál mac Peattair (c.1693–c.1770), Irish-language scribe, was the eldest of three sons of Peadar Ó Longáin, whose family were hereditary stewards of the monastery at Ard Pádraig before the reformation and subsequently stewards to the Fitzgeralds, the knights of Glin. Mícheál's nephew Tomás Ó Longáin was a Whiteboy and was transported to Botany Bay for his involvement in the 1798 rebellion. Ó Longáin spent his early years at Ballydonoghue, Co. Limerick. Nothing is known of his education. He was employed as a land agent and rent receiver to Edmond Fitzgerald, the last catholic knight of Glin. Fitzgerald lost his lands to his younger brother Richard in 1741 and Ó Longáin left Glin after a disagreement with the new knight. According to tradition Richard Fitzgerald cast Ó Longáin's manuscripts and books into the fire.
In the 1760s Ó Longáin settled permanently at Béal Átha Maghair in the parish of Carrignavar, Co. Cork. Colophons in his manuscripts indicate that theretofore he had moved about a good deal in Limerick, Kerry, and Cork. He also appears to have spent time in the capital. In a colophon to the Ossianic tale Cath Mhuighe Mucroimhe, completed in Dublin c.1752 for the catholic doctor John Fergus (qv), he referred to his continual wanderings from Glin in Co. Limerick. He came into contact with the Maigue poets whose poetry courts were held at Croom, Co. Limerick, and regularly took part in those courts. A ‘barrántas’ (warrant) issued by Aindrias Mac Craith (qv) against Seán Ó Tuama (qv) called upon Ó Longáin to give evidence at the court. Ó Longáin is referred to by Mac Craith as ‘Saor-fhear sár-ghlic sítheoilte, Aon den dáimh ba chaoine cáil, is groidhe-fhear grádhmhar gaois-ghleoidhte’ (‘A noble man, most wise and refined, one of the assembly of gentlest repute, and a man, sturdy, affectionate, in wisdom delightful’ (Dinneen, 116)).
Ó Longáin completed his first scribal transcription, a copy of the Chronicum Scotorum, in 1711 at Ballydonoghue from a manuscript he acquired from Sir Thomas Fitzgerald, knight of Glin. The next date recorded for him is 1740, when he was at Raheen near Castlehaven, West Carbery, at the house of Tadhg Ó Súilleabháin (qv). Twelve years later he copied Geoffrey Keating's (qv) Foras feasa ar Éirinn for Pádraig Mac Mathghamhna at Ballyadam, possibly in March 1752. In 1756 he was at Bandon, where he transcribed for Uileag de Búrc prose material contained in RIA, MS 24 A 15. In 1761 he transcribed a medieval medical treatise for Tadhg Ó hEidirsceoil in Coolmain, East Carbery. The following year he transcribed An Seanleabhar Muímhneach in Glanarought, Co. Kerry, for the catholic bishop of Cloyne and Ross, John O'Brien (qv). Ó Longáin may also have assisted O'Brien in the compilation of his Irish–English dictionary. Through his work for O'Brien he came into contact with Gaelic scholars from Cork such as Conchubhar Bán Ó Dálaigh and the Rev. Seán Ó Conaire, who worked closely with O'Brien during the 1760s. Evidence in Ó Conaire's and Ó Longáin's manuscripts also suggests that the two men worked closely together (for example, RIA, MS 23 C 23).
No information on Ó Longáin's wife is provided in any of his manuscripts. His son Mícheál Óg (qv) was born when he was seventy-three years of age and it is unclear whether he had any other children. He died around 1770–71 and was buried at Whitechapel cemetery, Co. Cork. His manuscripts are held in the RIA, NLI, the library of TCD, the American Irish Historical Society, New York, and the library of NUI, Maynooth.