Ó Neachtain, Tadhg (1671–c.1752), writer, scribe, lexicographer, and teacher, was born in Dublin, the first child among two sons and one daughter of Seán Ó Neachtain (qv), teacher and creative writer, and his wife, Úna Ní Bhroin, both of Dublin. He seems to have spent his whole life in the Liberties area of Dublin; at all events it is certain that he lived there for much of his adult life, at first in Coles Alley, from where he moved to Earl St. in 1709, taking a lease of seven years on ‘four street rooms’ (Ó Cléirigh, ‘Leaves from a Dublin manuscript’, 196), and that he had a school in the same street in 1731. He received at least some part of his education from his father, was fully literate in Irish and English, and had a competence in Latin, and in later life had an interest in history and geography. It would appear that it was as a teacher that he earned his living. He was a prolific scribe and may have earned some income from this work, but it is clear that he wrote several of his manuscripts for his own use.
He gathered around him a circle of scribes and scholars who sought to preserve what they could of the traditional Irish learning and literature, and with whom he exchanged books and manuscripts. One such manuscript, which was extremely important as a source of traditional historical material, was the Book of Ballymote, which in the first half of the eighteenth century belonged to TCD, and to which Tadhg Ó Neachtain in 1726 had access and which he may have had in his possession. From it he copied ‘Lebor Gabála Érenn’ (the book of conquests of Ireland) and the ‘Bansenchas’ (history of women). He also made copies of excerpts from ‘Lebor na Cert’ (the book of rights) and from a version in Irish of the tripartite Life of St Patrick, of a large collection of genealogies, of a significant quantity of bardic and Fenian verse, of part of an Irish translation of the medieval medical treatise ‘Lilium medicinae’, and of seventeenth-century texts such as ‘Foras feasa ar Éirinn’ and ‘Trí biorghaoithe an bháis’ by Séathrún Céitinn (Geoffrey Keating (qv)), and excerpts from Catechismus by Theobald Stapleton (qv). As a copyist of traditional material, it is evident that historical and religious texts were of great interest to Tadhg Ó Neachtain.
He was also an enthusiastic recorder of his own private experiences and contemporary public events in Ireland and abroad, making notes on what had come to his own attention and translating into Irish copious extracts from newspaper reports. Finally he, together with his friends, Seán Mac Solaidh from Stackallen, Co. Meath, Risteard Tuibear (qv) from Castleknock, Co. Dublin, and Muiris Ó Nuaba, a Munsterman, made copies of the original writing of Tadhg's father, Seán. Tadhg's holograph copies survive of his own writing, such as his geography of the world which he cast in the form of a conversation between himself and his father (ed. Meadhbh Ní Chléirigh; published as Eólas ar an domhan (1944)), his translation of the life of St Anthony of Padua (ed. Pádraig Ó Súilleabháin; published as Beatha Naoimh Antoine ó Phadua (1957)) and his poems, one of which, consisting of a 2,112-line history of Ireland from the beginning of time, he punningly informs the reader in the text that he, Tadhg Ó Neachtain, composed in 1726. He also wrote an extensive dictionary of the Irish language which has proved an important source to subsequent lexicographers; it is unusual in that the author was not content merely to record the contemporary lexicon of the language but also created many terms.
His verse is of documentary rather than literary interest. It provides an insight into the society in which Ó Neachtain lived and his personal concerns. It reveals him as being strongly Jacobite in sympathy and loyally Roman catholic in religious faith and on the other hand deeply antagonistic to the Hanoverian dynasty and to the reformed churches and sects. In several of his poems he provides information about his family and friends, listing the scholars in his circle, cataloguing his father's writings, commenting on the departure in 1728 of his son, Peadar, for Spain to join the Jesuit order, and expressing his sense of loss at the deaths of those who were close to him.
Tadhg Ó Neachtain survived at least until 1752. He married first (a.1709) Caitríona Nic Fheorais; they lived in Coles Alley and Earl St., Dublin, and had two children, Peadar (b. 29 June 1709) and Seán (d. 12 October 1714). After Caitríona's death (12 April 1714) Tadhg married Máire Ní Chomáin; she died in childbirth the following year (29 November) and their infant son, Pádraig, also died (26 November). He married thirdly (1717) Máire Ní Reachtagáin; after her death in 1733, he married Isabel Meares (d. 1745) of Mullingar, Co. Westmeath.