Leabhar na nGenealach ("Book of Genealogies") is a massive genealogical collection written mainly in the years 1649 to 1650, at the college-house of St. Nicholas' Collegiate Church, Galway, by Dubhaltach MacFhirbhisigh. He continued to add material until at least 1666, five years before he was murdered in 1671. It was published in a five volume edition in Dublin in 2004 as The Great Book of Irish Genealogies.
Described by Eoin MacNeill "by far the largest and fullest body of Irish genealogical lore", it contains roughly twice as much material as found in the Book of Ballymote and the Book of Lecan. It preserves notes on families from all parts of Ireland, Gaelic Scotland, the pre-Gaelic, Viking and Old English peoples of Ireland. It features an eighteen-page preface, nine 'books' or divisions and a seventy-four-page Clar or general index in double columns. It consists of eight hundred and seventy one pages, 95% of which is in Mac Fhirbhisigh's handwriting. The remainder is in the hand of an unknown amanuensis, and incorporates some pages written in 1636 by Mícheál Ó Cléirigh.
Many questions concern Leabhar na nGenealach. In the words of Nollaig Ó Muraíle:
" ... who or what prompted Mac Fhirbhisigh to undertake the compilation of Leabhar na nGenealach? ... how much planning and organisation (such as the collection of source material) preceded the writing of the book ..? Did he, at any time during the compilation ... entertain hopes of seeing it printed ...? Unfortunately, we have very little in which to base even the most tentative of answers to these questions. We simply do not know ..."
Nor is it known how he supported himself in Galway, though he did commissions for the Poor Clares and John Lynch while there. O Muraile suggests that it was a work compiled in his spare time, in between possible tutorial work for the children of local families (see The Tribes of Galway). Unlike the Four Masters, he appears to have had neither patron nor sponsorship of any sort.