Y-DNA analysis is a remarkable method that can inform patrilineal genealogies, both ancient, and modern. Applied here to facilitate a critical review of Branan pedigrees, an analysis of haplogroup R-BY140757 results in a deep questioning of the dominant narratives of the O’Braonáin Uí Dhuach (O’Brenan of Idough). What results is a disruption of those narratives that is total. The O’Braonáin Uí Dhuach, held by Ossorian historians to share descent from Cearbhall, King of Osraí (843-888 AD), we argue, are not Osraighe, but are an Uí Failghi tribe – this based on the ultimate authority of Dubhaltach Mac Fhirbhisigh. Hence, Y-DNA connections between Branans, or those with similar-sounding surnames, and related others, are a false trail for those who claim descent from Cearbhall. Once Mac Fhirbhisigh is embraced, and the erroneous pedigrees of the O’Braonáin Uí Dhuach are set aside, the origins of men with Branan, and similar-sounding surnames, of haplogroup R-BY140757, can be correctly determined. And, based on Y-DNA haplotype analysis, it is considered those origins are not with the O’Braonáin Uí Dhuach, or any Irish clan. Rather, haplotype R-BY140757 appears to have originated from a family who settled near Braham, in Suffolk, after the Norman conquest of England. The key figure in the appearance of R-BY140757 pedigrees in Éire is Sir Robert de Braham, who was Sheriff of Kilkenny ca. 1250 AD.