I know the subject of the lineage and DNA link to Niall of the Nine Hostages has been discussed at length over the last several years, but I wanted to let you folks know about a pretty good (and effectively succinct) article published on the subject just recently.  Here's an excerpt:

One thousand years ago this week, Brian Boru, High King of all Ireland, defeated NorseKing 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 theVikings 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.

You can read the rest of the article here.