The Genetic Genealogy of the peoples of Ireland and Great Britain had been the same from the end of the last Ice Age until the Anglo-Saxon period. Even today, 'The People of the British Isles' DNA study found Anglo-Saxon DNA generally in the southeast half of England with a maximum of 38% in the far southeast nearest the Continent. Recent DNA work has shown that there were three waves of migration into western Europe after the last Ice Age. There were people from the earlier time called the Paleolithic (Old Stone Age), but the ice cut their record off from us.
The waves began as the ice receded throughout Europe and the first wave into the Isles began around 8,000 BC. (There are remains of a hut from about 7000 BC at Mount Sandel, County Derry.) Mount Sandel, a Mesolithic Campsite. These Mesolithic (Middle Stone Age) people are called Hunter Gatherers and were not yet settled in a single permanent place. It was Western Hunter Gatherers (WHG) who made their way to Mount Sandel. (There are different labels for various related HG group across Eurasia.)
The second wave arrived in the Isles about 4000 BC and came from a culture called 'Old Europe' which developed in the Balkans and Greece about 6000 BC. The Old Europe people brought farming to Europe from Anatolia (modern Turkey) and had a very advanced culture for the time which became lost to history. Farming can support a much larger population than hunter gatherers and in the Isles, they replaced/absorbed the first wave people. Marija Gimbutas did early work in this area which is important for our story. These people are called Early European Farmers (EEF) and are classified as Neolithic or New Stone Age. Neolithic means that they farmed and/or kept livestock but did not work metals. Both the Egyptian pyramid builders in the Near East, and the Mayas and Incas in the Americas are classified as Neolithic. The famous megalithic monuments built across the Isles such as Stonehenge and Newgrange were built by these people who had an advanced knowledge of astronomy. Sometime after 3000 BC there was a population decline which paved the way for the next wave of people into the Isles.
The third wave brought metal working, cattle, chariots/carts, and war/draft horses into the Isles. It is their DNA that 'The People of the British Isles' DNA study found. The Indo-European languages arrived at the same time/place as them and while there is no definitive proof, it is generally accepted that they brough all of the modern European languages (except for a few) with them as they traveled across Europe beginning around 3500 BC when they moved into Old Europe from the Steppe lands. They mixed with the Old Europe people who had already mixed with the WHG people to form a new population. They came from the Pontic-Caspian Steppe which denotes the plains in southern Ukraine and Russia above the Black and Caspian Seas. (Pontus was the Greek word for the Black Sea.) We call these people the Yamnaya and we share much of their DNA.
Eventualy they moved west and north and came to populate all of the area from the Urals in Russia to the Rhine. The people from eastern Germany to the Urals were dominated by the male-line haplogroup R1a and called the Corded Ware Culture after their distinctive pottery. Those to the west by R1b which came to dominate all of western Europe, especially the Isles. Around 2500 BC, a new culture arose on the Iberian Penninsula and spread widely. This was a culture not a people but it was adopted by the western R1b peoples and we call it the Bell-Beaker Culture after their distinctive drinking vessels seen above.
The R1b people living in the Rhine Delta (The Netherlands), had a particular mix of the ancestry mentioned above which included elevated Yamnaya ancestry. It is these people who moved into the isles about 2400 BC. DNA results ahow that after about 100 years, they had almost replaced the Neolithic people and became over 90% of the population. (See the Single Grave Culture section below.)
The oldest Gaelic literature describes the origins of the Irish people as a series of ancient invasions, and the archaeological record in Ireland, as elsewhere in Europe, exhibits several horizons where major cultural shifts are apparent. The two most transformative are the arrival of agriculture (~3750 BC) followed by the onset of metallurgy (~2300 BC). The Neolithic package characterized by animal husbandry, cereal crops, ceramics, and timber houses reached the shores of Ireland some 5,000 years after its beginnings in the Near East. The second great wave of change starts with the appearance of copper mines, associated with Bell Beaker pottery, which are quickly followed by Bronze tool-making, weaponry, and gold-working, with distinct Food Vessel pottery succeeding from the earlier beakers. This period coincides with the end of the large passage graves of Neolithic Ireland in favor of single burials and smaller wedge tombs. (Cassidy 2015)
Dr Lara Cassidy of TCD authored an Irish paper which matches the work of her British counterparts. She took DNA from the remains of four skeletons from northeast Ireland and found results for Ireland which confirmed the British work. One of the specimens was a female from the Neolithic Age and three were male from the Bronze Age. All were taken from the northeast of Ireland as seen in the image. The female was from Ballynahatty and the males from Rathlin Island.
The 3 men were from around 1700 BC, 1800 BC, and 1900 BC and the woman was from around 3200 BC (see the image below). The male-only yDNA for the men was L21 which is still the most common in Ireland and is found almost exclusively in the Isles. Their aDNA (full genome) matches the present population of Ireland, so they were the earliest Irish people. (See this page for an explanation of genetic DNA).The woman's aDNA was very different as she was from the Neolithic population. Her closest relatives are the people of the island of Sardinia near Italy (see the heat map below). 'Otzi the Iceman' found in the Italian Alps and who died around the same time as the Ballynahatty woman is also closest to Sardinia. This does not mean that either of them came from Sardinia, but that Sardinia is a relic of the old pre-Indo-European population. So, we know that the present population of the Isles arrived around 2500 BC and began their journey on the Steppes, but how did they get here?
The way we learn about ancient DNA, its interrelationships, and its relationship to modern humans is by using full-genome or autosomal DNA (aDNA) data. DNA testing companies will return hundreds of thousands of SNPs. One of the Easton Hoy's kit has 650,420 SNPs and a method is used to compare his aDNA with ancient and modern samples to get a picture of the relationships. This method is called Principal Component Analysis or PCA and is a combination of Linear Algebra and Statistics. Most human DNA is alike, so we look for differences that people or groups have picked up over the years. PCA will give the combination of these SNPs that give the greatest variation among samples, and we pick the two best and re-map our data using these two new axies. The result is a PCA chart of the relationships among the tested. See the left and right images below from the Davidski - Eurogenes Blog. These graphs each have two axes labeled PC1 and PC2 which are the linear combinations of the hundreds of thousands of SNPs which give the most variation to separate samples on the graphics.
The David Reich Lab at Harvard is one of the main sources of ancient DNA and Reich has written an excellent book on the subject available at his site (also Amazon Kindle). The other is the Svante Pääbo at the Max Planck Institute, Leipzig.
These labs extract and analyze DNA known to be useful in classifying hominids. Svante Pääbo sequenced the first Neanderthal genome in 2009 and the first Denisovan genome in 2010. These labs make the data from their work public for further analysis.
The most useful for studying the ancestry of the Isles people is likely Davidski and his team at the Eurogenes Blog (see the link in the references below). This is their view supported by their own PCA analysis of the data from the labs.
It seems that the Bell-Beakers in the Rhine River Delta developed from the adoption of the Bell-Beaker package of artifacts from Iberia by the Single Grave Culture people in the Rhine Delta. DNA tells us that around 2500 BC, they moved en masse to the Isles and soon became at least 90% of the population. We know that the Bell-Beakers in the Isles had male yDNA of R1b, while the larger Corded Ware Culture had R1a. From the map above, we see that the CWC encompassed all eastern Europe to the Urals and then into Iran through northern India.
On the Steppe it is believed that the R1b people were to the south near the Black Sea and the R1a to the north near the vast Eurasian forest. At some point, they all moved west into the area of 'Old Europe' in the Balkans and the Carpathian Mountains. It seems likely that the southern R1b went south around the Carpathians to the Danube River and followed that until they found the Rhine and followed that to the sea and the Rhine Delta where they developed the Single Grave Culture and then the Bell-Beaker Culture. The more northernly R1a people would have gone north around the Carpathians and up the Elbe, Oder, and Vistula to the sea and filled in that vast area.
The spread of the Indo-European languages from Ireland to India, and their connections to the third wave Steppe riders is best explained in this book by David W. Anthony, "The Horse, the Wheel, and Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World published by Princeton University Press, First published July 26, 2010
Wikipedia has a thorough analysis of the book.