Mac Grianna, Seosamh (1900–90), writer in Irish, was born 15 January 1901 in Ranafast, Co. Donegal, one of eleven children of Féilimí Dhónaill Phroinsiais, farmer and fisherman, and his wife, Máire (née Ní Dhomhnaill). Both of his parents came from a storytelling background. His brother Séamus Ó Grianna (qv) was also an Irish-language author, and his cousin Domhnall Mac Grianna was an editor at An Gúm (the state-funded Irish language publishing house). Seosamh received his earliest education in Ranafast national school, where he became a class monitor. He began to write regularly in English at the age of fourteen. Most of his pieces were short, but one story about the first world war consisted of 7,000 words. On receipt of a scholarship in 1916 he moved to St Eunan's College in Letterkenny, Co. Donegal, where he remained for one year. He held strong republican beliefs during his youth and was expelled from St Eunan's for having in his possession ‘revolutionary documents unsuitable for a youth of his age’ (Ó Muirí, 47). Afterwards he spent three months in St Columb's College, Derry, but was asked not to return after Christmas for disciplinary reasons.
In 1919 he won the King's scholarship and entered St Patrick's College, Drumcondra, Dublin, where he trained as a national school teacher. On his graduation in 1921 he returned to Ranafast national school, where he remained for a year. During the war of independence he became a propagandist for the local IRA company but did not take an active part in fighting. Instead he wrote two plays, ‘An poitín’, and ‘Lag an tSeantí’, in 1921 as a fund-raising measure. With the outbreak of the civil war he fought in the same IRA column as his brother Domhnall in Donegal. In 1922, together with his brother Séamus, he wrote an anti-treaty drama entitled ‘Saoirse nó suaimhneas?’ Along with his brothers Séamus, Hiúdaí, and Domhnall he was captured by Free State forces during the summer of 1922 and interned in Newbridge internment camp, Co. Kildare. Both he and Hiúdaí were among a number of internees who went on hunger strike in November 1923. They remained in the camp until the end of the war.
Between 1923 and 1925 Mac Grianna had a number of temporary teaching jobs throughout the country, spending time in Tyrone, Dublin, Dundalk, Mayo, and Galway. He claimed to have taught in nine different counties. His difficulty in finding a permanent post may have been due to his republicanism but it may also have been related to his ability as a teacher. He disliked teaching, particularly teaching Irish. It is believed that he suffered a nervous breakdown in Co. Leitrim as a result of stress brought on by teaching. He never returned to the profession apart from taking part in summer schools in Omeath, Co. Louth, and Ranafast. He moved to Dublin, where he lived for a time with his brother Séamus. The brothers often disagreed with one another about literary and other matters, and it was not long before he moved out.
It was only in 1924 that he first began to write in Irish, and his writing career began in earnest after reading An Chéad Chloch by Pádraic Ó Conaire (qv). Much of his inspiration came from Ranafast, an area steeped in folklore. During 1924–5 he contributed many of his early short stories, including ‘Teampall Chonchubhair’, ‘Teacht Cheallaigh Mhóir’, and ‘Leas ná Aimhleas’, to the newly founded An tUltach. These later formed the basis of his first book, Dochartach Dhuibhlionna & sgéalta eile (1925). He also contributed numerous articles to An Camán, An Phoblacht, Fáinne an Lae, Humanitas, Inniu, Iris an Fháinne, Poblacht na hÉireann, Scéala Éireann, the Irish Press, the Irish Statesman, the Leader, the Standard, and the United Irishman. Although his active literary career only spanned approximately eleven years, he made an immense contribution to the development of literature in the Irish language, publishing ten original works, translating twelve books into Irish, and also publishing a substantial number of reviews and letters. His original works include Filí gan iomrádh (1926); a collection of short stories, An grádh is an ghruaim (1929); an historical novel, Eoghan Ruadh Ó Néill (1931); Pádraic Ó Conaire agus aistí eile (1936); An Bhreatain Bheag (1937) and Na Lochlannaigh (1938). Two works under one cover were published in 1940: Mo bhealach féin, a biographical novel, and Dá mbíodh ruball ar an Éan, which he had been unable to finish, stating in a footnote to the manuscript that in the summer of 1935 his creative ‘well’ had run dry. Although his final novel, An droma mór, which was awarded the Butler prize, had been completed in the autumn of 1930, it was only published in 1969. The manuscript had been with An Gúm from the early 1930s but they had failed to publish it, claiming that it contained libellous material. According to Ó Muirí, had the work been published when it had been completed it ‘could have, should have elevated him to being the foremost writer of his generation’ (Ó Muirí, 131). He also wrote an unpublished novel in English, ‘The miracle of Cashelmore’, in 1931. It is unknown what happened to the manuscript for this work. Another unpublished novel, ‘Réamonn Ó hAnluain’, is in private ownership.
He was employed as a translator by An Gúm, and it is estimated that he translated around one million words during the period 1926–34: Teacht Fríd an tSeagal (1932), An Mairnéalach Dubh (1933), Ben Hur (1933), Séideán Bruithne (1935), Muintir an Oileáin (1936), Teach an Chrocadóra, Díthchéille Almayer (1936), Ivanhoe (1937) and Imtheachtaí Fear Dheireadh Teaghlaigh (1936). Two further translations were published in the 1950s; Eadarbhaile (1953) and Báthadh an Ghrosvenor (1955). A new edition of Muintir an Oileáin was published by An Gúm in 1989 as an example of the best of the organisation's translations. Both he and Niall Ó Dónaill (qv), who also worked as a translator, held competitions to see who could translate the greatest number of words. At the end of one week Mac Grianna had translated 29,000 words of Ivanhoe as opposed to Ó Dónaill's 27,000 of The talisman. He paid a price for such frenzied work, however, and often had to take breaks from translating. By 1931 Mac Grianna had grown deeply critical of An Gúm's policies, particularly the organisation's bureaucracy and censorship, as he perceived it, and publicly aired his criticism in the form of letters to a number of newspapers. The strain of translating took its toll on his mental health, which failed again in 1935. This brought about the end of his writing career and he spent the period 1935–6 in Grangegorman Hospital, Dublin. From this time onwards he was forced to rely on his friends for financial assistance.
During the early 1930s Mac Grianna began cohabitating with Peggy O'Donnell; it is not clear if they ever married. They had a son together, Fionn, who was taken into care by the Christian Brothers in Glasnevin, Dublin, when his parents were unable to provide for him. Peggy committed suicide in 1959, and in the same year Fionn drowned himself in Dublin Bay. Around this time Seosamh was admitted to St Conall's Hospital, Letterkenny, where he remained until his death on 11 June 1990.