Ó hÉigeartaigh, Seán Sairseal (1917–67), civil servant and publisher, was born 11 February 1917 in Welshpool, Montgomeryshire, Wales, son of post office official, Patrick Sarsfield O'Hegarty, (qv) and his wife, Liaimín Nic Ghabhann, an Ulster presbyterian. Due to his nationalist activities, the British civil service had transferred P. S. O'Hegarty to Wales. The family returned to Dublin when Seán was a young child and he received his early education in St Bridget's national school and afterwards in St Andrew's College, where Séamus Caomhánach (qv) (Seán a Chóta) taught him Irish. He matriculated at TCD in 1939 where he became a foundation scholar and graduated with a first class honours B.Sc. degree. While in college he met his future wife, Bríghid Ní Mhaoileoin, also awarded the same degree. They married on 7 September 1943 and had two children, Cian Sáirséal and Aoileann Bríghid. While a student in TCD he was involved in the foundation of the Comhchaidreamh (the union of university Irish societies), was auditor of TCD's Cumann Gaelach (1939–40), and helped organise de Valera's (qv) first visit as Taoiseach to TCD in 1939. The following year the Cumann Gaelach demanded a ban on ‘God save the king’ being played at the graduation ceremony, arguing that it compromised Irish neutrality, an argument that was accepted by the board of TCD. Together with Seoirse Mac Aodhagáin, also an auditor of the Cumann Gaelach, he wrote a light drama entitled ‘Na Reachtairí Rógaireachta’, staged in the Peacock theatre, Dublin.
Ó hÉigeartaigh spent all of his working life as a civil servant, primarily in the Department of Finance but also in the Department of Supplies and the Department of Industry and Commerce. By 1948 he had become principal officer in Finance, and he developed particular expertise on pension matters. He also dealt with civil service mediation and arbitration, displaying particular skill in this area. He was involved in Ireland's preparations for joining the EEC.
Throughout his life Ó hÉigeartaigh was active in numerous Irish language organisations, and helped to establish several. He was a founding member of both Craobh na hAiséirí, a more militant branch of Conradh na Gaeilge, and Glún na Bua, which developed from Craobh na hAiséirí with its emphasis on the literary revival. While a member of Craobh na hAiséirí he wrote a pamphlet on C. S. Parnell (qv) with Roger McHugh (qv). When An Comhchaidreamh founded the monthly journal Comhar in 1942, he was appointed director and acted as editor for a number of years. The following year he was a founding member of Comhdhál Náisiúnta na Gaeilge and a member of the board of directors. As a result of his work, An Club Leabhar (Book Club) was established in 1949 and Bord na Leabhar Gaeilge in 1952.
In 1948, together with his wife he founded Sáirséal agus Dill, the first independent Irish language publishing company, with £300 he had inherited from his aunt, Éilís Dill Nic Ghabhann. His primary aim was to provide a forum for the new generation of Irish-language writers coming to the fore whose works An Gúm was unwilling to publish – people such as Seán Ó Riordáin (qv), Máirtín Ó Cadhain (qv), Séamas Ó Néill (qv), León Ó Broin (qv), Liam Ó Flaherty (qv), Annraoi Ó Liatháin (qv) and Máire Mac an tSaoi. The first work to emerge from the press on 30 November 1947 was Tonn Tuile by Séamus Ó Néill. Ó hÉigeartaigh was an innovative publisher and became an expert in the printing trade. By 1966 the company had published ninety books, including novels, dramas, poetry, short stories, school textbooks, diaries and correspondence and also had forty manuscripts in preparation. It was responsible for the publication of some of the finest literature in the Irish language: Cré na cille (1950) by Mairtín Ó Cadhain, Nuabhéarsaíocht (1950) by Seán Ó Tuama, Eirbeall spideoige (1952) by Seán Ó Riordáin and Ceo meala lá seaca (1952) by Micheál MacLiammóir (qv). This latter work was groundbreaking because it was the first book to be published in Ireland or England using a lithographic printing press. History was also made with the publication of Annraoi Ó Liatháin's Claíomh an díoltais (1961), the first work to be published by means of film print to be produced in either country. Ó hÉigeartaigh aimed for the highest possible publishing standards. The company had received £37,000 in state grants and was kept operating with the aid of family funds but by 1966 debts amounted to £11,000. He considered closing down at this stage but due to protests to the government by a number of writers, the minister for education, George Colley (qv), announced that the government would clear the company's debts and provide a monthly grant of £250 towards publication costs.
Ó hÉigeartaigh died suddenly 14 June 1967 in the Sáirséal agus Dill offices and was buried in Templeogue cemetery. After his death, his wife Bríghid, who had been responsible for the editorial side of the business, took over other aspects of the company. When the company finally ceased operating, Caoimhín Ó Marcaigh took over its copyright and goodwill and founded a new publishing house under the name Sáirséal-Ó Marcaigh to keep the most important works in print.