The Mac Costigan of Ossory: dismantling an assumed genealogy

There is nothing new about the narratives of some Irish clans being sometimes inaccurate. But there is a world of difference between slight exaggerations of deeds, or misdeeds, or colourful embellishments of a character’s prowess, or a clan’s conquering status, and an entirely false clan narrative. Yet, a critical review of Clan Mac Costigan of Ossory reveals a origin narrative far from the traditional. The dominant narrative of Clan Mac Costigan of Ossory over the past 140 or so years arrived in 1878 via Rev. John Shearman’s ‘Loca Patriciana’, in which he stated the Mac Costigan stemmed from ‘Oistegan’, a son of Seaffraid (Geoffrey) Mac Giolla Phádraig (d. 1269), who died in 1289. Sherman also stated that Oistegan’s son, Mac Oistegan, slew Thomas Butler, the First Baron Dunboyne, and 100 followers at Muileann gCearr, larmhí (Mullingar, Westmeath) in 1329. And so, the modern Mac Costigan narrative was born – yet it is an ‘assumed genealogy’, likely part inspired by Roger O’Farrells’s ‘Linea Antiqua’ (1709), and then encouraged along by the ever-unreliable John O’Hart. But the attribution of Butler’s death to a Mac Costigan is hopelessly wrong. In addition, not a single ancient Mac Costigan pedigree exists. Rather, the Mac Costigan of Ossory arose in the mid-fifteenth century, as a Killaloe clerical lineage. And it is John Mac Costigan, who later took the alias Mac Giolla Phádraig, who takes centre stage in this article’s early stanzas. The co-use of the surnames Mac Costigan and Mac Giolla Phádraig in fifteenth century Ossory is also captured in the current era because some Costigans and Fitzpatricks (formerly Mac Giolla Phádraig) share a common ancestry under haplotype R-A1488, and the Time to the Most Common Ancestor (TMRCA) of R-A1488 is ca. 1420. The TMRCA speaks to the very obvious, and it is impossible, based on either historical records or DNA analysis, to determine whether R-A1488 Fitzpatricks descend from a line of Mac Costigan, or vice versa. It is little wonder then, that throughout the sixteenth and seventeenth centuries, the Ossory land possessions of the Costigan, Fitzpatrick, and other kindred clans who had also adopted obscure aliases, such as the Mac Fynen (also known as Mac Kynen), notably in the Parish of Offerlane, were adjacent, interchanged, and shared. Here we dismantle the ‘assumed genealogy’ of the Mac Costigan of Ossory and posit an alternative based on robust genealogical research and twenty-first-century science.