the Derbhfhine

For almost 1,000 years, the main unit of Gaelic society was not the nuclear family as we conceive it, but a very particular version of the extended family, the derbhfhine (true kin), all the descendants of a common great-grandfather. 

Among other things, property ownership rested with the derbhfhine, not the individual. So what you could own – cattle in particular –  depended on who your kin were. No wonder genealogy loomed so large and surnames that signaled kinship were so important.

 "The ancient Irish were among the earliest civilizations to value genealogy. "Those of the lowest rank among a great tribe traced and retained the whole line of their descent with the same care which in other nations was peculiar to the rich and great ", noted John O'Donovan in Miscellany of the Celtic Society, "for, it was from his own genealogy each man of the tribe, poor as well as rich, held the charter of his civil state, his right of property in the cantred in which he was born, the soil of which was occupied by one family or clan, and in which no one lawfully possessed any portion of the soil if he was not of the same race as the chief." (Genealogy and Brehon Law)

In his post How Gaelic surnames were Englished Irish Genealogist John Grenham notes........."The name you bore was transparent to those around you, not just, as today, a convenient marker, but instead laden with resonance: stories, possessions, reputations, feuds, homeplaces . . . Gaelic surnames were deeply ingrained in everyday social interactions, as vital and ordinary as language or weather or food." 

So....if we date ourselves back to a common great-grandfather we would find Thomas Madden and our derbhfhine could contain our our Madden, Marqueling and Thornton cousins as well as our Madden second cousins.

 Genealogy and Brehon Law
The Structure of Early Gaelic Society - The Druid Network (this offers a great explanation of the Irish family system)
Derbfine- Wikipedia