If you have a Niall of the Nine Hostages badge in the Badges section of your dashboard, this means that you match exactly or are a close match to the historic Irish Modal Haplotype (IMH). The IMH was documented in a 2006 Y-chromosome population genetics study.
In 2006, a group of researchers explored the frequency of haplogroup R-M269 and the Western Atlantic Modal Haplotype (WAMH) in Ireland. They showed that haplogroup R-M269 accounts for 85.4% of the lineages in Ireland, but that a distinctive haplotype is found there at a frequency of 8.2 to 21.5%. The authors attribute this Y-chromosome signature to Niall of the Nine Hostages, a medieval warlord.
Niall of the Nine Hostages received his name from the taking of hostages as a strategy for playing mental havoc upon his opponent's chieftains. He is known in folklore as a raider of the British and French coasts. Supposedly slain in the English Channel or in Scotland, his descendants were the most powerful rulers of Ireland until the 11th century.
Niall's descendants were called the Uí Néill. It is their prolific production of progeny to which the journal article's authors attribute the modern frequency of Niall's haplotype. The authors state:
Gaelic society placed great emphasis on family relationships organized around a strongly patrilineal system (derbhfine) in which land and title could be handed down to successors chosen from within a kin group of male lineage relatives. This wider inheritance cohort resulted in a decreased likelihood of dissociation of lineage from power (O'Croinin 1995). Also, whereas medieval Ireland was Christian, earlier marriage customs persisted and allowed divorce and concubinage.
For your match with the haplotype of Niall of the Nine Hostages, FamilyTreeDNA uses our Y-12 marker set and allows for a single mutation, i.e., a genetic distance of 1. Notably, in our database, Niall's signature is from .6% to 1.0% of our male customers.
(O')Neill, (O')Gallagher, (O')Boyle, (O')Doherty, O'Donnell, Connor, Cannon, Bradley, O'Reilly, Flynn, (Mc)Kee, Campbell, Devlin, Donnelly, Egan, Gormley, Hynes, McCaul, McGovern, McLoughlin, McManus, McMenamin, Molloy, O'Kane, O'Rourke, and Quinn.
Moore, L., B. McEvoy, E. Cape, K. Simms, and D. Bradley (2006, February). A y-chromosome signature of hegemony in gaelic ireland. The American Journal of Human Genetics 78 (2), 334-338.