The first Irelanders

3D reconstruction of the crania of Louschbour man,
a WHG (Western Hunter Gatherer) who lived around 6000BC
in what is now Belgium, Irish WHG would've looked more or less like this.

Ten thousand years ago, or twelve depending on who you ask, a small band, probably no more than a couple dozen, dark skinned, blue eyed people made landfall on a small island for the first time ever to our knowledge. 

Only a few millennia before the island had been an arctic wasteland that was completely inhospitable to human habitation. Covered in glaciers which gradually melted during the Bølling–Allerød oscillation resulting in the hilly landscape seen here today. The Allerød oscillation resulted in warmer temperatures in the North Atlantic similar to modern levels, before things eventually got cold again, even colder than LGM levels in fact. This cold period is known to us as the 'Younger Dryas' and would've roughly coincided with the arrival of these hunter gatherers to the island we know today as Ireland.

Their ancestors would have originated somewhere in the vicinity of modern day south eastern Europe, belonging to the Epigravettian complex. Rather than the previously hypothesised Franco-Cantabrian regium hypothesis, with the dinosaur Palaeolithic continuist nonsense of R1b cromagnon supermen colonising everything, it seems that Iberia, which was for the latter half of the Upper Palaeolithic a bastion for some of the first Anatomically modern human populations to arrive in Europe during the Aurignacian, however these UP survivors themselves it seems did not survive the expansion of the Epigravettian foragers from the Balkans, with samples from the Epipaleolithic Azilian harbouring significantly higher levels of WHG related genetic ancestry than those hunters of the preceding Magdalenian. The fact that Basques today exhibit the highest WHG genomic ancestry is as it turns out a mere coincidence with virtually all but one Mesolithic and Neolithic samples belonging to paternal lineages other than R1b, such as G2a, I2a and H2a, with the lone exception from El Trocs cave with one farmer belonging to now extinct (except in Sardinian men) R1b-V88. 

This lineage likely made its way into early farming communities from Anatolia somewhere in the Balkans where it was found in Mesolithic samples from Lepenski Vir, Serbia, and the Epigravettian Villabruna individual, the oldest known WHG yet uncovered. 

Front and lateral view of Ripari
Villabruna's skull, oldest as of
yet known individual to possess
the R1b haplogroup, as well as 
blue eyes.
14kya.

Magdalenian hunters (El Miron) already harboured significant levels of WHG ancestry 
as early as 20,000BC WHG had expanded across the Mediterranean. 

With the extinction of most megafauna towards the end of the Pleistocene, early Holocene foragers such as the WHG became dependant on smaller game, with red deer, Irish elk, boars, wild aurochs and rabbits for meat. An emphasis on freshwater fish was also made with most Mesolithic sites being located next to river lanes. As a result of this, WHG were substantially shorter than hunter gatherers that hunted the megafauna that proved plentiful during the UP. With the average Gravettian male standing at 1.83m WHG men on average stood at a much shorter 1.63m. Though were powerfully built.

From Grasgruber et al 2014

For the first time ever we now have two samples from the Mesolithic era from Irish Western Hunter Gatherers, and rather intriguingly they appear to be somewhat inbred, although not to the same extent of that of the "god king" individual buried at Newgrange almost to the point of forming their own unique clade from continental and even British WHG. From the supplement of Cassidy et al 2020: 

"We note a significant decrease in inbreeding through time in ancient populations. Hunter-gatherers show both the largest variance in inbreeding coefficients and the highest median. However, none are estimated to have parents who are more closely related than four degrees. This demonstrates that the maintenance of outbreeding mating networks was common practice in the Upper Palaeolithic and Mesolithic periods of Europe, as already seen for the Sunghir site in Russia."

These people knew about inbreeding, and the problems that would have resulted from it.

The samples were taken from from the remains of two male individuals from Killuragh in Limerick and Sramore in country Leitrim. They both lived around the tail end of the Mesolithic roughly 4000BC. Just as the first wave of farmers were beginning to colonise the island from the Atlantic façade.

After four to six thousand years of isolation they had become so genetically drifted from continental WHG to the point of almost being a separate lineage entirely. At its zenith the largest Mesolithic population would have been no more than a few hundred individuals. Which would explain why Irish Neolithic samples were largely identical in terms of WHG admixture to continental counterparts.

Like the vast majority of WHG the uniparentals for these two were fairly typical. Both belonged to subclades of mtDNA haplogroup U5b and Y-DNA haplogroup I2a. With Killuragh6 belonging to I2a1a2 and Sramore62 to I2a1b2, the same haplotype as that of the most famous WHG, Cheddar man. Who made the headlines in 2018 for a somewhat subpar reconstruction.

 A more recent and quite frankly 

probably more accurate reconstruction 

compared to that created by 

the Natural History museum of London.

Artist’s impression of a mesolithic Iberian from
La Braña, Arintero from around 5000BC
just as the first wave of cardial farmers arrived
from across the Mediterranean. 
Basques today have amongst the highest proportion of 
WHG admixture in Europe.

Killuragh cave, a site which appears to 

have been frequented by mesolithic hunters

for thousands of year with many 

bone fragments being recovered from here.

Rather then being buried within the cave systems, bones were placed at the mouth of the cave, perhaps for ritualistic reasons which were then washed away by natural processes. 

                                                                                                                                       

"Closest" modern populations

As per the conclusion of Cassidy et al 2020, the two Mesolithic individuals shared highest drift with British and other Epigravettian offshoot populations from the continent. 

3D PCA

2D PCA

When modelled with Epigravettian WHG (Villabruna)
Irish samples do not score any Iberian HG as previously thought

As for introgression by Irish WHG into oncoming neolithic communities that established themselves on the island roughly around 4000BC, this signal seems to be nonexistent as the vast majority of Neolithic samples from the island do not harbour significantly higher levels compared to contemporaneous samples from the continent, specifically the cardial Mediterranean region where we now know they spread from. 

Genocide?

Given that the foragers would've numbered no more than a few hundred, and the farmers in the tens of thousands, it's logical to think that Irish WHG more or less met their end by the beginning of the Neolithic transition, as did most WHG did in general with some straggler groups surviving in northern Scandinavia in isolated pockets as recently as 2000BC.

For the majority of or at least the first half of its populated history, as much time has transpired since the beginning of the Neolithic to now as has the beginning of the Mesolithic to its end, if we take the twelve thousand year date into consideration.

Conflicts over land probably did occur, with local WHG probably hunting imported domesticated livestock and sparking the occasional scrap, but I don't see any reason to suspect any semblance of a violent genocide, with peaceful integration probably occurring over several centuries as cultural boundaries between the two began to fade as it would do again in the later Bronze age. 

Cosy