His best-known work is the Leabhar na nGenealach, which was published in 2004 as The Great Book of Irish Genealogies, by Éamonn de Búrca, more than 300 years after it had been written.
Mac Fhirbhisigh was most likely born at the family castle, in the parish of Lackan, Tireragh, County Sligo, sometime in the first quarter of the 17th century.
At some point between 1643 and early 1645, Dubhaltach moved to the town of Galway, where in April 1645 he completed a transcription of the seventy-page historical-genealogical compilation called Seanchas Síl Ír.
On 8 December 1647, Dubhaltach noted that he had completed the translation – from English into Irish – of part of one book and all of a second concerning the Rule of St. Clare, and related matters.
Nollaig Ó Muraíle describes it as "a compilation of Irish genealogical lore relating to the principal Gaelic and Anglo-Norman families of Ireland and covering the period from pre-Christian times to the mid-17th century and collected from a variety of sources.
That August saw him complete a catalogue of the Kings of Ireland, from Partholón to Ruaidrí Ua Conchobair, entitled Réim Ríoghraidhe Éireann.
Of Dubhaltach's whereabouts and activities in this period, Ó Muraíle has the following to say: "At one time most of the citizens ... had to abandon the stricken town and settle temporarily in the surrounding countryside.
"[2]In fact, Dubhaltach's only remark thought to be connected with this time is what Ó Muraíle calles a "breathtaking understatement" that Dubhaltach writes in the Díonbhrollach (preface): Ma ta aoínní inbéme ann seacha sinn, iarruim are an tí fhéudas a leasughadh, go ttuga Dia duinn airthearrach uaine (as suaimhnighe ina an aimsir-si) / If there is anything in it deserving of censure apart from that, I ask him who can to amend it, until God give us another opportunity (more peaceful than this time) to rewrite it.
In July, these forces, under Sir Charles Coote, began a nine-month siege of Galway which ended in the town's surrender in April 1652.
In July 1653 at an unknown location, perhaps still in County Galway, he added further material, along with a separate index to the book's list of Saint's pedigrees.
[4] In April 1656, Dubhaltach acted as a witness to his hereditary lord, Dathi Og O Dubhda (David O Dowd) upon his marriage to Dorothy O Down.
After this he disappears until around 1662 when he is named in an official report as liable to pay hearth-tax for a dwelling in Castletown, just a few miles north of his native Lackan.
From the archives of the Clan MacFhirbhisigh he drew upon documents and sources to write a tract, in English, on early and medieval Irish bishops.
Nothing more is known of Mac Fhirbhisigh until January 1671, when his friend Ruaidhrí Ó Flaithbheartaigh noted in a manuscript: "1670/1 mense Janu: Dualdus Firbisius obiit, a Thoma Croftono occisus."
Mac Fhirbhisigh was stabbed to death by local man Thomas Crofton, at a shebeen near the village of Skreen, County Sligo.
The circumstances of the killing were related by Eugene O'Curry as follows:...the last of the Mac Firbiscs was unfortunately murdered at Dunflin, in the county of Sligo, in the year 1670.... Mac Firbis was, at that time, under the ban of the penal laws, and, consequently, a marked and almost defenceless man in the eye of the law, whilst the friends of the murderer enjoyed the full protection of the constitution.
[5]According to Ruaidhrí Ó Flaithbheartaigh – as noted by Edward Lhuyd – Dubhaltach's manuscripts were passed on his death to his lord, David O Dowd.
Leabhar na nGenealach next came into the hands of Séamus Bacach Mág Uidhir of An Leargaidh (Dowra-Blacklion area) who made a copy in 1715 or 1716 at Stranamart, north-west County Cavan.
Following his death and that of his son, Dr. Fergus's daughter put his extensive library up for auction at their home in Abbey Street, Dublin, on 3 February 1766.
For much of the next hundred and fifty years, it was placed at the Jocelyn family home of Tollymore House, Newcastle, County Down, who were in part descended through a female line from the local Magennis clan.
During this time extensive use of Leabhar na nGenealach was made by Charles O'Conor (1770s), Thady Connellan (1830s), John O'Donovan (MacCarrick's version) and Eugene O'Curry, who transcribed in between March 1836 and February 1837.
During the 1690s, one "Dudley Forbissy, Ardneere, clerk, commonly called Prior of the Abbey of Ardnaree" appears on a list of persons "Outlawed for Foreign Treason".
In 1842, a letter (dated 15 August 1842, Dublin) was received by the Royal Irish Academy from one John Mac Firbis, a farmer, "in a humble state of poverty," from the parish of Lackan, County Sligo.
O Muralie suggests that as the letter was written in Dublin, Mac Firbis and his family may have been seeking financial aid while emigrating from Ireland.
Under the Anglicised surname Forbes, descendants of the Clan MacFhirbhisigh are still to be found in small numbers in north Mayo, mainly in and about the town of Ballina.
Without the great diligence, then, of Dubhaltach Mac Fhirbisigh, as copyist, compiler and translator, and also as collector and transmitter of manuscripts, some quite significant remnants of the civilization that was Gaelic Ireland would have gone into almost certain oblivion.