In April 1649 Mac Fhirbhisigh was working on what would come to be considered his magnum opus, Leabhar na nGenealach, or the Book of Genealogies. 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." The fact that many of these sources no longer exist adds considerably to the value of Mac Fhirbhisigh's work. This is particularly true of items held at Lackan by the Mac Fhirbhisigh's.
It is not known how long Dubhaltach spent collecting the necessary materials and planning the book's layout. Nor is it known when he commenced writing; he does note that on the "13 April, in Galway, 1649," he had completed a fifty page tract on the genealogies of the Ui Bhruin down to "do shlioch Brian mc Eathach Muighmheadhoin/the lineage of Brian son of Eochaidh Muighmheadhoin." That August saw him complete a catalogue of the Kings of Ireland, from Partholón to Ruaidrí Ua Conchobair, entitled Réim Ríoghraidhe Éireann. He names the source of this material as Leabhar Airisean Fhearghail Uí Ghadhra, an alternative name for the Annals of the Four Masters. Exactly how this work reached Galway has never being explained.
Réim Ríoghraidhe Éireann was completed on 8 August 1649, just as the Bubonic plague entered Galway on a Spanish ship. Over the next nine months it killed some three thousand seven hundred of the town's inhabitants. 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. This must have caused our scholar grave inconvenience. For example, if he did move to rural base, how much of his precious source-material - the various 'old books' to which he sometimes refers, all too often with maddening vagueness - would he had to carry them with him? And how did he transport them? Did he travel on foot or on horseback? Did he have an assistant? Alas! These are just some of the questions to which he gives us not the slightest hint of an answer."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 five us another opportunity (more peaceful that this time) to rewrite it."With most of the text compiled, Dubhaltach added an index of just under three thousand entries, an index being rare in a Gaelic manuscript. This was completed on the Feast of the Holy Innocents (28 December 1650) just as English parliamentary forces, completing the Cromwellian conquest of Ireland, crossed the Shannon. 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. As in earlier periods of his life, Dubhaltach's activities in this period are unknown.
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. Ó Muraíle identifies one of his probable sources for this material as "one of two early-12th-century manuscript-recensions of the work known as the Irish Liber Hynorum, while another was the great early-15th-century manuscript now known as the Leabhar Breac (formerly Leabhar More Duna Daighre)." As the latter seems to have been held during the length of the 17th century at the Franciscan convent of Ceineal Fheichin, Abby, County Galway, it may hold a clue to Mac Fhirbhisigh's location at some point after the fall of Galway.
Read more about this topic: Dubhaltach Mac Fhirbhisigh