Regarding DNA... although I can trace my Great Great Grandfather to Elgin, Scotland, my ancestral YDNA is Norse. Specifically, SNP R-FT40329. I asked a lot of questions about this and was in turn invited to become Co-Admin of the Ross surname project at Family Tree DNA. So, have been studying and learning how to analyze YDNA STR and SNP data.
First, I found that my subgroup of Ross men have distant patrilineal cousins with the Matheson surname, who hail from Loch Alsh, Scotland. This is just south of the Applecross peninsula. The significance of this is that in 1217, the Earl of Ross was known to be the son of the lay abbot of the Applecross monastery and was also recorded as being of the O'Beolan family.
Men with Ross surnames are generally able to trace their own genealogy to near Inverness; the ancestral lands of Clan Ross in the Highlands of Scotland with a terminal SNP R-FT40329.
Men with the Matheson surname are generally able to trace their ancestry to Loch Alsh on the Western side of the Scottish Highlands, just south of the Applecross peninsula. These men tend to be downstream of SNP R-FT60163. The Ross and Matheson men have a common ancestor living around the year 1150 AD (950-1350 AD).
Isle of Man men are downstream of R-BY30287 while UK and Irish men are downstream of R-S7678. The Isle of Man men and the UK and Irish men have a common ancestor living around the year 750 AD (600-900 AD).
The common ancestor for all of these men lived in area of modern day Norway around 50 AD (200BC - 250 AD). Everybody in in this Norse-Gaelic group is downstream of R-S7680, who migrated about 2,300 years ago from near the Danish Island of Zealand to Norway. That migration occurred at the beginning of the Iron age. Ancestors appear to have resided in region of Norway for about 1100 years.
The split between the Ross/Matheson men and the Manx/Irish/UK men occurred within Norway. One group eventually migrated to Scotland while the other group migrated to the Isle of Man, Ireland and the UK. There are no other groups of men within the R1a haplogroup on the Isle of Man other than those mentioned above.
Our common ancestor with Somerled, the mid-12th-century Norse-Gaelic lord, is R-Z284, who lived about 4,600 years ago near the Danish island of Zealand.