Mac Firbhisigh, Dubhaltach Óg (c.1600–71), antiquary, was a native of Lackan, parish of Kilglass, Co. Sligo. His father, Giolla Íosa Mór Mac Firbhisigh, was a member of a family that had provided the O'Dowds, lords of Tír Fhiachrach (the barony of Tireragh, Co. Sligo), with hereditary seanchaidhe (antiquaries or chroniclers) since the twelfth century; his mother, whose given name is not known, was a daughter of Eoghan Gruama Mac Diarmada, a member of a junior branch of the MacDermots, lords of Magh Luirg (the baronies of Boyle and Frenchpark, Co. Roscommon), that had settled in Tireragh. Dubhaltach Óg Mac Firbhisigh had three brothers, but it is not known if he had any sisters.
He was well educated, as befitted his family's background and social status, and acquired a familiarity not only with traditional Irish learning but also with Latin and English, and he had some knowledge of Greek. There is evidence to suggest that he may have been educated at either the school of the Mac Aodhagáins – another of the hereditary learned families of Gaelic society – at Ballymacegan, Co. Tipperary, or at a school associated with the Jesuits in Galway city, or indeed at both establishments. It is also likely that he worked as a teacher himself, but the earliest reliable information about his life dates from 1643.
In that year Mac Firbhisigh completed a manuscript at Ballymacegan. In the same year, and probably at the same place, he completed his first major work, the annalistic compendium entitled Fragmentary annals of Ireland in its most recent modern edition, which he copied from a source that has since been lost, for the use of Dr John Lynch (qv) of Galway city. Mac Firbhisigh moved to Galway a short time later, where he resided in the collegial school attached to the church of St Nicholas and made an Irish translation of the Rule of St Clare for the use of a congregation of Poor Clare sisters. It seems likely that the collection of annals now known as Chronicon Scotorum was copied at Galway during the years 1649–50, and it is possible that this text may also have been copied for John Lynch.
Mac Firbhisigh's most substantial work, a book of genealogies incorporating material drawn from the Book of Uí Maine and the Leabhar Breac, among other sources, dates from the same period and may originally have been intended for publication. A notable feature of this compendium is the extensive space devoted to the genealogies of Anglo-Norman families, reflecting the alliance of Old Irish and Old English communities in contemporary politics. Mac Firbhisigh returned to Co. Sligo either shortly before or after the Cromwellian capture of Galway. In 1665, and again in 1666, he visited Dublin where he was employed by Sir James Ware (qv) to make English translations of Irish material from sources which included the Book of Lecan, the Annals of Innisfallen, and the Annals of Tigernach. By 1666, Mac Firbhisigh was living at Castletown, in the parish of Easky, less than 10 km from his birthplace, and it is likely that he spent the final years of his life there.
It is not known if Mac Firbhisigh married or had any children. In January 1671 he was fatally stabbed by a drunkard named Crofton in a tavern at Doonflin, parish of Skreen, Co. Sligo.