The families are arranged within their respective ethnic groups by tribe, sub-tribe and clan. Implicit here is the understanding that each of the five ethnic groups of Gaeldom fostered related tribal populations, and that these tribal populations comprised the basic political and social structure of Ireland and the Scottish Highlands until the seventeenth and eighteenth centuries.

Every Gaelic tribe originated in one or another of the five ancestral ethnotribal population groups of Gaeldom, hence the division of Part II into five chapters. The charts which precede each chapter show the relationship of each tribal branch to one of the five euhemerized Celtic ancestor-deities traditionally linking the tribes of that particular ethno-tribal group. Sub-tribal branches of an independent and geographically isolated nature are branched independently on the chart but are linked in the text. The position of the tribes on the chart is generally indicative of their relative locations within Gaeldom.

Sometimes confusion arises in clan and tribal nomenclature, as such names often acquired a double meaning as territorial designations. These names are, in this book, used in their original tribal sense. The tribal and clan names are in Gaelic, the names of the families are given in their translated form, in English. As such, the sept (i.e., clan/family) names given generally represent the main form used with Gaelic prefixes ("0" and "Mac") although there are often a great variety of Anglicized forms extant. "0" and "Mac" denote descent from the person whose name follows, e.g., the forms "MacDonald" (literally "son of Donald") and O’Brien (literally "grandson of Brian") when they are employed as family names, are used in the general sense of marking descent from those individuals. Translations were accomplished in three ways; either by meaning (e.g., "0 Sionnaigh" in Gaelic became "Fox" in English), or by phonetic approximation (e.g., "0 Cearnaigh" in Gaelic became "O’Carney" in English), or by "attraction," in which case a family’s name was translated (by them or for them) by using a common English name of roughly similar sound (e.g., "0 hUiginn"—O’Higgin—became "Higgins").

Regarding tribal and clan names, these also indicate descent: "cineal," "clann" and "corca" generally translate as meaning the progeny or kindred of the ancestor whose name follows. Similarly, "dal" means "tribe of," "muintear" means "family of," "siol," seed or progeny, "ui," grandsons or descendants, and so forth. Likewise, terminal affixes such as "-acht," "-na," "-ne," "-raighe" in. dicate descent from the name which precedes. "Fir" or "feara" means "men of," and is used in clan names which make reference to territories.

As for the families and the area and time covered, with the exception of a few merchant families, and some Anglo-Norman families around Dublin, the entirety of Gaeldom in 1500 was under the political dominance of the families dealt with in Part 11. As a genealogical note, it should be stated that descent from these families is a thing to be particularly proud of, for these were the chiefly families whose actions molded the history of Ireland and Scotland. For such families, a code of honor went hand-in-hand with their royal or noble status, and was a major force in the Gaelic ethos, though there were of course exceptions. Family standards of ability and conduct were set generation by generation, and such kin groups were expected, as a matter of blood, to live up to the precedents set by their ancestors and maintain or advance the family’s honor and position within the Gaelic tribal aristocracy. Such is the stuff of history.

These Gaelic aristocratic families tended to be very prolific, having large families and often producing children by mistresses as well. As a result, there tends to be a redundancy of patrilineally-traced royal blood in Gaeldom, as men of the commoner sort tended to lose out in the numerical contest of fatherhood, especially over time.