Mac Fhirbhisigh's lengthy introduction specifies the contents, author, time and place: "Cuimre Cráobhsgaoileadh cineadh ó Adhamh gus anois, 1666, tionóiltear a leabhraibh Cloinne Fhirbhisigh (is go hairidhe as an leabhar do sgriobhsam fén go foirleanthan are cráobhsgaoileadh mór – agus mion-bhabhal Ereann in gach am) egarthas agus sgriobhthar sonna lesin Dubhaltach mc Giolla Íosa Mhóir mc an and Dubhaltach Mec Fhirbisigh Lecain Mhec Firbhisigh i tTír Fhiachrach Muaidhe, anno Christi 1666: anius and dara lá is an cédlúan do mhís Aibreóil [le toil nDé) trosaigheam so isin Tír Fhiachrach remebeartha/An Abridgement of the genealogical ramifications of the peoples of Ireland and the Scots of Alba together with their principal genealogical branches from Adam until now, 1666, which is assembled from the books of Clann Bhirbhisigh (and especially from the book I wrote myself at length on the ramification of the great and minor branches of Ireland at every time) and is arranged and written here by An Dubhaltach son of Giolla Íosa Mhóir son of An Dubhaltach Mec Fhirbisigh of Lecain Mhec Firbhisigh in Tír Fhiachrach of the Muaidhe in the year of Christ 1666; today, the second day and first Monday of the month of April, we begin this (by God's will) in the year and Tireragh aforesaid.Having written Leabhar na nGenealach, Mac Fhirbhisigh took on the task of making a smaller concise version of the text.
O Muraile writes (on page 267 of The Celebrated Antiquary): "In a development which is quite unique to this work – not being found, at least in such a systematically developed form, in any other Irish genealogical collection – ... the more important genealogies are set out in the reverse of the usual order of son: father: grandfather, and so on; instead they begin with a more or less distant ancestor and progress from father to son to grandson and so on down – in many cases – to a figure more or less contempoarty with Dubhaltach.
(p.269, Ó Muraile, 1996)"Dubhaltach's rate of progress can be gauged by a colophon dated Saturday, 5 May 1666 – at a point where he had written some 45% of the Cuimre since he began on 2 April.
At some point later in the year Dubhaltach returned to Dublin, where he translated annalistic material for Sir James Ware.
On Saturday 1 December 1666 Sir James died, five days after his seventy-second birthday, bringing their collaboration, and Dubhaltach's employment in Dublin, to an end.