Ó Súilleabháin, Tadhg ‘Gaelach’ (O'Sullivan, Timothy) (1715?–1795), poet, was probably born in the parish of Killeedy, about 8 km south of Newcastle West, Co. Limerick. Nothing is known about his immediate family or early life, but his Irish translations of prayers in the Latin breviary have given rise to speculation that he may have been a student for the priesthood. He arrived in Co. Cork about 1740 and over the next thirty years lived at various places in the east of the county, including the Midleton, Rathcormac, Carrignavar, Cove, Cork city, and Youghal areas. It is likely that he worked as a teacher throughout his life. There is no evidence that he married.
Ó Súilleabháin's early work is typical of Munster poetry of the period, being a mixture of romantic verse, laments and eulogies for members of the catholic gentry and clergy of east Cork, and Jacobite compositions. The latter include aislingí such as ‘Ar maidin inné dham is déarach do bhíos-sa’ and ‘Ar maidin ag caí dham go fann táir’, and drinking songs such as ‘Tógaidh bhur gcroí, bídh meidhreach meanmnach’ and ‘Cúrsa na cléire agus ciúine na héigse’. Ó Súilleabháin was friendly with Seán Clárach Mac Domhnaill (qv) and Fr Liam Inglis (qv), two prolific authors of Jacobite verse, but his Jacobitism was not confined to the field of literature and he was imprisoned for publicly drinking the health of Prince Charles Edward and the Old Pretender in Cork city. The poem beginning ‘Mo shlán leatsa, a Éire’ appears to have been composed while he was in gaol, and the song ‘Cé dealbh dúbhach mo scéimhchruth’ praises a Domhnall Ó Faoláin from Ardagh, Co. Limerick – close to the poet's native district – who was instrumental in securing his release.
Sometime in the 1760s Ó Súilleabháin moved to Dungarvan, Co. Waterford, and about the same time he had a profound religious conversion as a result of which he turned his back on secular literature – a decision that was deplored in verse by the Co. Clare poet Tomás Ó Míodhcháin (qv), who also praised Ó Súilleabháin's learning and mentioned his knowledge of Latin and Greek. Ó Súilleabháin continued to compose verse but for the rest of his life he appears to have concentrated entirely on religious topics. He produced a series of finely crafted poems in simple but elegant language – many of which were set to popular airs – on such devotional themes as the persons of the Holy Trinity, the Virgin Mary, chastity, the rosary, and the patron saint of the Decies, St Declan. It is unclear when Ó Súilleabháin left Dungarvan, but he can probably be identified with the ‘T. Sullivan’, a teacher of Greek, Latin, and book-keeping based at Ballinakill in Queen's Co. (Laois), who advertised in 1779 for subscriptions to finance the publication of a history of Ireland called ‘Antiquitas Hibernica’ that he had translated from the original Irish. No such work appeared however, and Ó Súilleabháin spent the final years of his life in Waterford city.
According to tradition, Ó Súilleabháin collapsed and died while praying in a Waterford church on 22 April 1795. He was buried in Ballylaneen cemetery, about halfway between Waterford and Dungarvan. His friend and fellow poet Donnchadh Rua Mac Conmara (qv) composed an epitaph in Latin that was inscribed on a headstone erected in 1910. Ó Súilleabháin's religious verse had circulated in manuscript form before his death, and in 1802 a printed anthology containing twenty-five of his religious poems was published at Clonmel under the title Timothy O'Sullivan's Irish pious miscellany. Between 1816 and 1879 more than a dozen editions of the Pious miscellany appeared at Clonmel, Cork, Limerick, and Dublin, leaving little doubt that it was the most widely read Irish-language publication in the period before the foundation of the Gaelic League.