South-West Scotland is a mysterious part of the country, which has been excavated less than other areas and about which very little is known historically outside the Kingdom of Alt Clut (Dumbarton), which later became Strathclyde. Its genetic profile stands out from the the rest of Scotland though because of a specific branch of haplogroup R1b-L21; the most common Scottish, Irish and Welsh haplogroup since the Bronze Age.
This branch is known as 'DF109' and is otherwise most common in Ireland, specifically the provinces of Ulster and Connacht1. Now, this distribution is complicated by the fact that Scots migrated en masse into Ulster in the 17th century, so most DF109 in Northern Ireland is likely from Ulster Scots (indeed, it is one of the most common haplogroups among them as the main branch of M222, as shown in this graph: 2).
Regardless, if we exclude Northern Irish samples, DF109 is most found in North-West Ireland, specifically County Donegal. While some Ulster Scots did settle in Donegal, there were much less than further east, which means most of the DF109 there is probably native. So then one is left to wonder: how did a paternal lineage with a strong association with North-West Ireland end up in South-West Scotland? Indeed R1b-DF109 makes up a sizeable chunk of L21 in South-West Scotland and suggests a mass influx of Irishmen at some point in history.
A map of R1b-M222 in Ireland with associated surnames, 'Higgins' comes from 'Viking'
The answer may lie in the very poorly-documented Viking Age. At the time, Strathclyde was a Welsh-speaking kingdom (though their language is distinguished from that spoken in Wales with the label 'Cumbric'), which may have actually seen a period of expansion south into what is now Cumberland in the early 10th century3. This followed the crumbling of the Anglian Kingdom of Northumbria, which happened as a result of the Danish conquest of Yorkshire in the 9th century.
When the Norsemen invaded the Hebrides and Ireland in the 9th century, they intermingled with the locals and the Gaels began to refer to them as Gall-Gáidhil, meaning 'Foreign Gaels'. While they clove to the Catholic faith and spoke the language of the Gaels, they had at least partial Norse heritage, and even when the Normans arrived in Ireland in the 12th century, distinctions were made between the Irish with and without Norse ancestry; with those having some Norse blood occupying a middle position between the Anglo-Normans and the 'pure' Gaels.
Though history is silent on the matter, we can tell from archaeology that Norse-Gaels were settling on the Irish Sea coast of Britain. However, it is only in South-West Scotland, specifically Strathclyde and Galloway (named after the Gall-Gáidhil) that R1b-DF109 is so common. As a kingdom, Strathclyde was strong enough to fend off the Northumbrians and maintain her independence prior to the 9th century, and important enough that her king (either Dyfnwal ap Owen or his son, Malcolm) met with Edgar the Peaceful of England along with six to eight other kings (including Kenneth II of Scotland) in 973.
However, by the 12th century Strathclyde was no longer an independent kingdom, but part of Scotland. Galloway persisted as a quasi-independent kingdom, though nominally subject to Scotland. The boundaries between these two realms were not always as they are today and there is a good deal of evidence that much of what is now the Strathclyde coast was part of Galloway in this early period. Ayrshire and Renfrewshire did not fall within a 12th century inquest of the Diocese of Glasgow, which could imply these areas had been part of Galloway (though by the 12th century, it seems that the areas themselves had been taken by Malcolm Canmore along with the rest of Strathclyde in the previous century)4.
Norse-Gaelic areas are yellow, though the ones south of Argyll most likely came from Ireland
If the Strathclyde coast was under Galwegian control for some time, this could explain how Malcolm was able to conquer Strathclyde if the kingdom was already weakened by conflict with the Norse-Gaels. The strong presence of R1b-DF109 in these areas suggests this was driven by mass immigration from Ireland, perhaps specifically from Dublin where a sample carrying this haplogroup has been found5. These men may have descended by slaves taken from North-West Ireland by the Dublin Vikings.
Another haplogroup we could easily associate with the Norse-Gaels, though much less common that R1b-DF109, is called R1a-YP609. The particular branch of R1a this haplogroup belongs to (Z284) is much more common in Norway than other Germanic countries, so we can securely connect it with Norse heritage. YP609 has been found in Ayrshire and Dumfriesshire6, which corresponds well to the areas settled by Norse-Gaels. There are other haplogroups associated with Germanic peoples, but these are often not so specifically Norwegian and so it would be hard to tell apart Norse-Gaelic lineages from Anglo-Norman ones which colonized the Scottish Lowlands from the 12th century onwards.
Examples such as this show us that we can use the study of genetics to help fill in the gaps of history, though it is still often guesswork and it's always better to confirm it with archaeological samples. The reason I think South-West Scottish R1b-DF109 doesn't come from 19th / 20th century Irish immigrants is because its prevalence among Ulster Scots suggests it was brought from Scotland, which would then mean there must have been an earlier migration from Ireland. It makes sense that there has been much back-and-forth between Ireland and South-West Scotland, as that area of Scotland is the part of Britain closest to Ireland.
Conor Cummings
1https://www.yfull.com/tree/R-DF109/
3Clarkson, 2016: Strathclyde and the Anglo-Saxons in the Viking Age, pp.63-69
4Clarkson, 2016: Strathclyde and the Anglo-Saxons in the Viking Age, pp.153, 164 & 165
6https://www.yfull.com/tree/R-YP609/