O'Sullivan, John Marcus (1881–1948), historian and politician, was born 18 February 1881 at Killarney, the second son of M. O'Sullivan, JP, merchant. He was part of an extended O'Sullivan family grouping, which struggled for dominance of East Kerry politics with the rival Murphy faction. Eugene O'Sullivan, who won the East Kerry seat from the sitting MP John Murphy in January 1910 but was unseated on petition because of intimidating behaviour (which included boasting of his family connection with ‘the Moonlighters of Firies’) was a relative; O'Sullivan's elder brother, Timothy (d. 1950), represented East Kerry at Westminster (1910–18). His uncle Charles (with whom he was on close terms) was bishop of Kerry (1918–27).
O'Sullivan was educated at St Brendan's College, Killarney, at Clongowes Wood College (1897–8), where he was a senior grade exhibitioner, and at UCD, where he was a contemporary of James Joyce (qv) and co-founded a student philosophical society, the Academy of St Thomas Aquinas, with Professor William Magennis (qv). O'Sullivan won the gold medal of the Literary and Historical Society for public speaking in 1902–3; he was a member of the Young Ireland Branch of the UIL. He was also active in the college sodality and the Society of St Vincent de Paul, and won a prize in religious knowledge. In 1902 he took first-class honours in the BA with first place in philosophy, and in 1903 the MA with special prize. He won a studentship in philosophy in 1904, and in 1906 a junior fellowship in philosophy of the RUI. These awards allowed him to travel to Germany for further study at the universities of Bonn, Berlin, and Heidelberg, where he received a doctorate with the highest possible distinction. His dissertation, ‘Vergleich der Methoden Kants und Hegels auf Grund ihrer Behandlung der kategorischen Quantität’, was privately published by the University of Heidelberg. During his stay in Germany O'Sullivan went on several walking tours with his friend Tom Kettle (qv) (then studying at Innsbruck), which gave him an abiding knowledge of, and love for, the German countryside, especially the Rhineland.
On his return to Dublin in 1908 O'Sullivan translated his dissertation as Old criticism and new pragmatism (1909). He applied for the chair of logic and philosophy at the new UCD, but was defeated by Father John Shine (d. 1960). Instead he was offered the chair of modern history, which had been his subsidiary degree subject. As professor, O'Sullivan was primarily a teacher rather than a researcher (his department produced only four MAs during his forty-year tenure of the chair). Although he was regarded as an inspiring lecturer and contributed widely to such journals as Studies and the Irish Ecclesiastical Record, his projected books on Rousseau, the Enlightenment, and Machiavelli never advanced beyond extended lecture notes. He saw Luther, Machiavelli, the men of the Enlightenment and the French revolution, Marx, and Lenin as ‘prophets of disorder and disunion’ and believed that the achievements of European civilisation could only be safeguarded by a renewed social order based on regained catholic faith and Thomist realism. O'Sullivan regarded himself explicitly as an engaged catholic scholar, denouncing intellectual and political ‘neutralism and indifferentism’; his contributions to relatively highbrow catholic journals coexisted with more populist apologetics in the Irish Catholic, the Irish Rosary, and pamphlets published by the Catholic Truth Society of Ireland. His form of philosophical history is best compared with that of his fellow catholic intellectual and Fine Gael politician James Hogan (qv).
O'Sullivan took little active role in the war of independence (though W. T. Cosgrave (qv) later stated that the underground dáil cabinet sometimes met in his house on Morehampton Road). He supported the treaty, and in 1923 he was elected Cumann na nGaedheal TD for Co. Kerry. Friends believed that he would have preferred to concentrate on academia, but saw it as his duty to obey the call to public life. His 1923 lecture to the central branch of Cumann na nGaedheal, ‘Phases of revolution’ – later published as a pamphlet – expresses the conservative view of the civil war as representing a nationwide moral collapse and the Cumann na nGaedheal government's role as saving the country from anarchy through firm and capable administration. Though initially somewhat lacking in confidence, O'Sullivan soon established a reputation as one of the country's most formidable parliamentary speakers, able to read up and expound the most diverse subjects. (His multi-tasking abilities extended to marking examination papers during dáil debates.)
As parliamentary secretary to the minister for finance, with responsibility for the Board of Works (1924–6), O'Sullivan achieved a reputation as a capable administrator of national reconstruction; he carried the important Arterial Drainage Act for the River Barrow through the dáil (and in the process displayed greater knowledge of the Barrow than local deputies). In January 1926 he succeeded Eoin MacNeill (qv) as minister for education. Here he again showed himself an able administrator, though his scope for action was limited by his firm belief that primary responsibility for educational matters lay with the churches. His major legislative achievement, the 1930 Vocational Education Act, was severely constrained by clerical insistence (to which he willingly acquiesced) that the new vocational schools should not herald state encroachment on the churches’ role and should not permit their professional status to rival the established secondary schools; as a result vocational education rapidly acquired a reputation as inferior. O'Sullivan's achievements as minister for education were also affected by his other duties. He represented Ireland at the League of Nations, attending the assembly as a delegate in 1924, 1928, 1929, and 1930; he served as a rapporteur on public health in 1928 and as president of the fifth commission of the league in 1930. He was chairman of the Cumann na nGaedheal party from 1927 in succession to J. J. Walsh (qv), where his role was as much to prevent the party grass roots from challenging the leadership as to extend and maintain the organisation. He also acted as go-between in negotiations between Cosgrave and the catholic archbishop of Tuam over the appointment of Letitia Dunbar-Harrison as Mayo county librarian.
O'Sullivan's political rise reflected the progressive displacement of the protectionist and populist Sinn Féin element within Cumann na nGaedheal by a more conservative and administratively oriented group linked to Clongowes and UCD. After the defeat of Cumann na nGaedheal in 1932 O'Sullivan remained one of the opposition's most prominent spokesmen in the dáil, but was able to take a more active role in university life. His first three-year term on the governing body of UCD had been hampered by his belief that he should not participate in its meetings after his appointment to a government position for fear of conflicts of interest; now he served two further terms on the governing body and was also elected to the NUI senate. After the division of Kerry into two constituencies came into effect in 1937, O'Sullivan represented North Kerry until his defeat in 1943, after which he retired from politics. He continued his academic duties, but his powers as a lecturer and his ability to organise material intended for his projected books had deteriorated.
With his wife, Agnes Crotty, O'Sullivan had two sons and two daughters, for whom he built a dolls’ house with electric lighting, which was later donated to the Peamount sanatorium for invalid children to play with. He also grew his own vegetables. From 1933 O'Sullivan, who inclined to obesity, suffered from hypertension, and friends believed that constant travel between his Dublin home and his constituency further undermined his health. He died 9 February 1948 at his home in Orwell Road, Rathgar, from a stroke.