Large-scale DNA studies focusing on Ireland are seeming to appear almost back-to-back. Last December in Scientific Reports, Gilbert, O'Reilly, Merrigan, et al. published "The Irish DNA Atlas: Revealing Fine-Scale Population Structure and History within Ireland." From that abstract:
The extent of population structure within Ireland is largely unknown, as is the impact of historical migrations. Here we illustrate fine-scale genetic structure across Ireland that follows geographic boundaries and present evidence of admixture events into Ireland.
Less than two months later, we had a new peer-reviewed article published by Ross Byrne, Rui Martiniano, Lara Cassidy, Matthew Carrigan, Garrett Hellenthal, Orla Hardiman, Daniel G. Bradley, and Russell L. McLaughlin in PLOS Genetics: http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1007152.
Titled "Insular Celtic Population Structure and Genomic Footprints of Migration," this study used haplotype-based fineSTRUCTURE (an algorithmic tool that can identify fine subdivisions in populations by using dense sequencing data) and identified 23 discrete genetic clusters that had geographic localization. The study found the clustering most pronounced in the west of Ireland, and less so in the east where historical migrations diluted older haplotypes. Similarly, and as expected, they found a cline of Celtic-British ancestry moving from the west coast of England toward the east.
Two other things stood out to me on a quick reading. First, they found Irish genetic proximity in all Scottish samples which, quoting the study, "likely reflects older strata of communication across the narrowest inter-island crossing." Historically, this isn't surprising. In the 4th century, as far as we know, only the Picts lived in what is today Scotland, and they lived mainly in the east, in areas north of modern Edinburgh. Its etymology unclear, the Romans used "Scoti" as a term for all Gaels; in other words, the Irish. In 297 the Roman's documented an attack along Hadrian's Wall, in which the Picts had the help of Scoti allies, indicating that travel from Ireland to the southwest coast of Scotland was already frequent. With the closest point of the island of Ireland and Scotland's Mull of Kintyre separated by only 13 miles, it's quite realistic to believe that travel between the two had been taking place long before arrival of the Romans.
At the beginning of the Early Christian period, Ireland was comprised of well over 100 chiefdoms, sometimes described as "petty kingdoms." One of these, Dál Riata, was on the island's northeastern corner, nearest to today's Rathlin Island. Sometime around 390-420 AD, the Dál Riata began to settle across the strait in what is now County Argyll. They brought their language with them. The families of Insular Celtic languages are the Goidelic (Irish, Scottish Gaelic, Manx) and the Brittonic (Welsh, Cornish, Breton). That coastal area of Scotland is rugged, and was lightly populated by the Picts. The Dál Riata, the Scoti, probably faced very little opposition for the land. For the next few centuries, the Scotis and the Picts alternately fought amongst themselves, and cooperated to raid and harass the Romans and the Anglo-Saxons. Some speculate that competition from the Norse and Angles probably contributed to the unification of Scots and Picts into a single kingdom in the early- to mid-9th century. Pictish language and culture became extinct, and the Roman moniker Scoti stuck and became Scotland.
Second, the study concludes that the genetic influence of the Norse-Vikings is greater than previously thought from considering yDNA haplotypes alone.
Of all the European populations considered, ancestral influence in Irish genomes was best represented by modern Scandinavians and northern Europeans, with a significant single-date one-source admixture event overlapping the historical period of the Norse-Viking settlements in Ireland. This was recapitulated to varying degrees in specific genetically- and geographically-defined groups within Ireland, with the strongest signals in south and central Leinster (the largest recorded Viking settlement in Ireland was Dubh linn in present-day Dublin), followed by Connacht and north Leinster/Ulster. This suggests a contribution of historical Viking settlement to the contemporary Irish genome and contrasts with previous estimates of Viking ancestry in Ireland based on Y chromosome haplotypes, which have been very low.
From a population genetics perspective, at first blush Ireland seems like one of Europe's best test tubes: a sizable population before the arrival of the Roman Empire; not isolated, but at least physically separated by ocean from from its English and Scottish neighbors; and possessing a language distinct from the roots of its three major visitors: the Romans (Italic > Latino-Faliscan > Latin), the Anglo-Saxons (Germanic > West Germanic > Anglo-Frisian > Anglic), and the Vikings (Germanic > North Germanic > Old Norse > Old East Norse).
But in broad studies, population geneticists can only work with what they have in contemporary timeframes; large volume samples can't be obtained other than from living or recently living people. Enter the Irish diaspora. One thing Ireland is known for over the course of the last 300 years is the Irish leaving Ireland. Since the early 18th century, around 10 million people born in Ireland have emigrated elsewhere. That's more than the total population of Ireland at its highest level, in 1840 when it numbered 8.5 million. The event I think most people associate with Irish emigration is the "Great Famine" between 1845 and 1849. That's one reason 1840 saw the largest population count. During the four years of the famine, around one million people starved to death, and an equal number fled to other countries. Ireland's population decreased by 20% to 25% virtually overnight.
Opinion only, but it seems a broad population genetic study of Ireland in, say, the late 1600s, might have revealed a different picture than uniparental testing recently has. That's why NextGen sequencing of autosomes is beginning to provide a different glimpse of past populations. If members of a family carrying an ancient Gaelic yDNA haplogroup left Ireland in 1846, never to return, uniparental DNA testing in Ireland 160 years later might never identify that haplotype.
And it isn't just improvements in autosomal sequencing that's casting new light on previous research. Published late last January in the European Journal of Human Genetics, Lan-Hai Wei, Shi Yan, et al., performed full-sequence Y-chromosome analyses disproving that the yDNA haplogroup previously referred to as the "C3*-Star Cluster" was directly associated with Genghis Khan and his descendants. Instead, NextGen sequencing revealed that the yDNA signature believed to be the Khan's is actually common in a diffusion of several Mongol-speaking populations.
It's staggering how much genetics for genealogy and anthropology have developed since the completion of the Human Genome project. The first mappings were done and that project closed in April 2003, and the pace of change—fueled by hardware, software, research, and public interest—has done nothing but increase every year since.