One thousand years ago this week, Brian Boru, High King of all Ireland, defeated Norse King Sitric Silkenbeard of Howth at the Battle of Clontarf. Brian Boru is a hero to many Irish people and widely revered for his great achievements.

Historical records tell us that, among other things, he ended the Uí Néill clan’s 500-year reign as high kings of Ireland, and also, through his victory at Clontarf, kicked the Vikings out of Ireland. However, we have an independent record of the past in our genes. We can look to patterns of genetic variation here to ask if Brian Boru was effective on both of these counts.

The Uí Néill clan trace their origins to the perhaps mythical Niall of the Nine Hostages. Niall was supposed to have lived 500 years before the Battle of Clontarf. Using genetics it is possible to trace Niall’s DNA and measure his legacy in terms of how many descendants he left. We can’t go back to AD 500 for a DNA sample, but we can look at modern O’Neills.

Ireland has one of the oldest surname traditions in the world. Also, whereas in other countries names reflect professions or townlands, Irish surnames refer to ancestors. Traditionally, surnames are passed from father to child. Barring adoption and other cases, the handing-down of this outward symbol of family is mirrored exactly by the genetic transmission of Y-chromosomes from fathers to sons. This genetic inheritance forms an unbroken chain from the past to the present.

In a survey of Y-chromosomes of Irish men, Prof Dan Bradley of Trinity College Dublin showed a small number of Y-chromosome types predominate in Ireland. In particular, one of these Y-chromosomes is very common in the northwest, being found in about one in five men there.

The close genetic relationship of these Y-chromosomes to each other suggests a single origin – one or more dominant males. This geographic area coincides with the ancestral seat of the Uí Néill family. Could this be the genetic trace of Niall of the Nine Hostages?

Irish Families by Edward MacLysaght (my grandfather) provides genealogical origins and grouping of surnames. When Y-chromosomes of O’Neills and other Uí Néill surnames are analysed, they nicely group together genetically and share the dominant Y-chromosome type.

This genetic evidence gives independent support for the authenticity of historical and genealogical records. But it is also strong evidence for long-lasting dominance of a single male lineage in medieval Ireland. Although Niall of the Nine Hostages may not be a historical personage, the Uí Néill clan was clearly dominant.

So, much like the hegemony of Genghis Khan, which has left a legacy of 10 per cent of men in the region of Mongolia sharing a single Y-chromosome, and almost 1 per cent of all men worldwide, Niall of the Nine Hostages has left a strong genetic imprint on modern Ireland.



Boru of the Banner

Brian Boru was from Clare and of the Dál Cais clan. Genetic evidence accords with historical and genealogical records, which say that O’Briens and Kennedys are close relatives belonging to this clan. However, in Munster there isn’t the same pattern of a single dominant Y-chromosome, and thus no evidence of a single dominant clan, despite Boru’s prominence. However, that is not surprising when you consider the Uí Néills had a 500-year head start.

But what of the Vikings? The Vikings (or Norse) founded many of our cities, including Dublin, Limerick and Cork. But what is their genetic legacy?

Bradley and colleagues looked at the Y-chromosomes of Irish men with surnames of Norse origin. If these Norse surnames are direct descendants of the Norse invaders of 1,000 years ago, then these Y-chromosomes should group with Norwegian Y-chromosomes.

However, the analysis showed the contrary. The Y-chromosomes of Irish men with Norse surnames group closely with Irish Y-chromosomes. This suggests that, despite their influence in establishing urban settlements, Vikings didn’t leave a dominant genetic legacy. The genetic evidence matches the historical story.

Ireland has experienced major upheavals, political dynasties, and migrations right up to today. These are recorded in patterns of genetic diversity that provide a window into the past and show the lasting legacy of leading characters such as Niall and Brian Boru.



Aoife McLysaght is a professor in genetics in Trinity College Dublin, where she leads a research group focus ing on identifying and interpreting the evolutionary patterns in animal genomes