Mac Airt, Seán Breandán (Arthurs, John Brendan) (1918–59), Irish-language scholar, was born 16 May 1918 in Keady, Co. Armagh, one of eight sons and two daughters of James Arthurs, owner of a hardware shop, and Mary Arthurs (née Nugent). One of his brothers, Eugene Arthurs, was bishop of the catholic diocese of Tanga, Tanganyika. Mac Airt received his early education at the De la Salle College in Keady and won a scholarship to St Patrick's College, Armagh. He graduated from QUB with a BA in 1939 and won a travelling scholarship that allowed him to visit Dublin, where he became acquainted with the leading Irish scholars Osborn Bergin (qv) and T. F. O'Rahilly (qv). In 1941 he was awarded an MA by QUB for his thesis, ‘The verbal system of bardic poetry’. In 1942 he joined the staff of the School of Celtic Studies at the Dublin Institute for Advanced Studies and remained there until 1948 when he returned to Queen's, where he was head of the department of Celtic for ten years.
Mac Airt's first major publication was an edition of the bardic duanaire or poem-book of the O'Byrnes of Wicklow, which appeared under the title Leabhar Branach in 1944. His edition of the Annals of Inisfallen – the first full edition of the oldest surviving Irish annals – was published in 1951. He was the prime mover in the establishment of the Ulster Place-Name Society in 1952 and edited the society's Bulletin, as well as contributing regularly to scholarly and Irish-language journals. In 1952 he began to prepare a modern edition of the first section of the Annals of Ulster (ending in 1056) as a doctoral dissertation for the NUI; however, his teaching and administrative duties at Queen's delayed progress and he died before finishing the work. The broad range of his scholarly interests is indicated by his intention to write a book on Irish toponomy and a dictionary of Ulster Irish – plans which were frustrated by his death at the age of 41.
Mac Airt died at Belfast on 19 June 1959 after an illness lasting nearly a year. His edition of the Annals of Ulster, extended to the later date of 1131, was completed by Gearóid Mac Niocaill (1932–2004) and published in 1983.