To put it concisely, Bede was a monk living in the North of England in the 700's. He wrote many works, including HISTORIAM ECCLESIASTICAM GENTIS ANGLORUM: LIBER PRIMUS. This contains the line, Scottorum nationem in Pictorum parte recipit, which some people believe refer to the arrival of the Scotts in Pictland from Ireland.

Bede also speaks of the Britons living in Scotand, but everyone seems to ignore that, just like they ignore that the Romans referred to a Scottish tribe as the Scotia in the first century AD. Where Bede got this info from, we don't know. It was unremarked upon until the Medieval period of Scottish History, where it was used to tie in a claim by some royalty/aristocrats/Clan leaders to tie in with the bloodline of Brian Bóruma mac Cennétig, High King of Ireland. Earlier Europeans had done similar family trees to claim descent from Alexander the Great.

A link was made to a notation in the Irish Annals of Tigernach, which states Feargus Mor mac Earca cum gente Dal Riada partem Britaniae tenuit, et ibi mortuus est. (Fergus Mór mac Eirc, with the people of Dál Riata, held part of Britain, and he died there.) However this section of the Annals appears to have been written about 500 to 600 years after this supposed event, and it's been made doubtful that such names existed in their written forms at that earlier time.

Fergus is noted in other books giving him a long lineage, some saying he was decended from King Arthur. Other sources say he was allowed into Britian to help Arthur battle the Picts. The lineage doesn't mention the likes of Ceretic Guletic of Alt Clut, who was a real Scottish king in the 5th century.

However, in 16th century George Buchanan in his Rerum Scoticarum Historia added much, much more to the legend. By now Fergus was fighting the Romans, the Franks and the Picts before becoming King of Scotland. He was eventually killed in battle against Durstus, king of the Picts, and was succeeded by his son Eugenius. However Buchanan says that the Scots came from Scandanavia.

However, the Royal lineage from "Fergus, 1st King of Scotland", was used as proof of Royal blood for a log time, James VI was a "happie Monarch sprung of Ferguse race" and in the Great Gallery at Holyroodhouse in Edinburgh there is a series of 89 paintings of the line of kings from Fergus to Charles II of England. It's supposed to be an unbroken line, but there is no basis for the tale. As well as legend placing him as both Irish and Scandanavian, the book of Lismore has a character called Senán son of Gerrgenn who appears to have shared a lot of the events in the life of Fergus.

Today, with archeological and genetic findings, it's all pretty much scattered to the winds. I am no expert on this or anything, I just read magazines.