From the book, "Scotland: a Concise History" revised edition, by Fitzroy Maclean, 1993, page 21:
The divisions and disagreements of the four kingdoms of Scotland [the Picts, the Scots, the Britons of Strathclyde, and the Angles of Lothian and Northumberland] weakened their resistance to the common enemy-the Norsemen. It was not until the 9th century that some measure of unity was at last achieved. In the year 843 Kenneth MacAlpin, King of the Scots of Dalriada and at the same time a claimant to the Pictish throne, a man, we are told, 'of marvellous astuteness', fell upon the Picts, to whose ruling dynasty he was related, after they had been weakened by the raids of the Norsemen, and, having disposed of all rival claimants, made himself King of everything north of the Forth. From Dunadd he moved his capital to Forteviot in the heart of [page 22] Pictish territory while the religious centre of his kingdom was shifted to Dunkeld. Thither he now transferred St Columba's remains from Iona. 'And so', says the Huntingdon Chronicle, 'he was the first of the Scots to obtain the monarchy of the whole of Albania, which is not called Scotia.' Of the Picts, who had ruled over most of Scotland for more than a thousand years, little or nothing more was heard. Right up to his death in 858, Kenneth MacAlpin sought repeatedly to conquer the Angles of Lothian. But in this he was unsuccessful. His successors, involved as they were in continuous warfare with the Norsemen, fared no better.

From the book "The Oxford History of Ireland" edited by R. F. Foster, 1992, ISBN 0-19-285271-X, page 7:
By far the most successful Irish colony in Britain was that of Dál Riata in Scotland; it lasted and finally laid the basis for the kingdom of Scotland. The Dál Riata or at least the group of dynasties to which they belong are located by Ptolemy in the extreme north-east of Ireland. When and why they crossed over to Scotland is uncertain. Medieval Irish legends tell that this began in the 3rd or 4th century, or indeed before. Other traditions state that Fregus Mór mac Eirc and his brothers established Dál Riata in Scotland about the middle or late 5th century. Whatever its beginnings, the Scottish kingdom of Dál Riata was a great success and by the time Columba came on his mission to Iona in 563 the king of Dál Riata was extending his authority over the Picts to the east. In the middle of the ninth century, Dál Riata took control of all Pictland, and Scotland became a united kingdom under Kenneth mac Alpine.

Notes Weis' "Ancestral Roots. . ." (170:13).
The following is taken from an Internet posting of Michael R. Davidson of Edinburgh. Scotland, on 23 Oct 1995:
IV. The Fall of the Picts, and the Rise of the Kings of Scots
In 789, the branch of the Cenel nGabrain represented by the descendants of Fergus mac Eochaid took control of the kingship of the Picts. For the next fifty years, with the exception of one three year period of joint kingship, they had a monopoly on its power. Since Fergus' descendants also had a claim to the kingship of Dal Riata, there are also several examples of individuals holding the two kingships subsequently or even simultaneously. The Pictish kingship was the gem of eighth and ninth century Scotland, and the success of the descendants of Fergus in holding it marginalized other branches of the Cenel nGabrain. It probably explains why the children and grandchildren of Aed Find, which represent the line we are concerned with, are so obscure. It is probable that they would have disappeared from recorded history were it not for the disaster which was to befall the descendants of Fergus, here recorded in the _Annals of Ulster_ for the year 839:
The heathens (i.e. Vikings) won a battle against the men of Foirtriu, and Eoganan son of Oengus, (the king of the Picts) Bran son of Oengus, Aed son of Boanta (the king of Dal Riata), and others almost innumerable fell there.
Eoganan and Bran were grandsons of Fergus, and their death probably left the descendants of Fergus without a viable candidate to the kingship, leaving a power vacuum in Scotland. This presented a golden opportunity for other dynasties to assert their position, and the sons of Alpin were all too happy to seize the moment.
Two notes on terminology:
It is from Kenneth mac Alpin that historians begin to number the kings of Scots, and employ Anglicized versions of their names. I will, however, give their names in their Irish forms in parentheses after their name and number. The proper title for the mac Alpin kings is the 'King of Scots'; they were king of the people, not of the land. This title was not, however, employed until the beginning of the tenth century. From 848 to 900, the mac Alpin kings retained the title 'King of Foirtriu' or 'King of Picts'. For this period, they are probably most accurately described as 'Kings of Picts and Scots'.
The text of this section is primarily drawn from Smyth's _Warlord's and Holy Men_, and Hudson's _Kings of Celtic Scotland_.
Kenneth I (Cinaed mac Alpin)
Within a year after the death of his cousins in 839, it appears that Kenneth seized the kingship of Dal Riata. There is little historical record from the next eight years, but it appears that Kenneth followed in the footsteps of his cousins, and made a bid for the kingship of the Picts. He was resisted, ineffectually, by a short-lived dynasty bearing Pictish names. Later legends suggest that Kenneth achieved his success through treachery; slaying his Pictish guests at a feast. Whatever his means, Kenneth defeated his last Pictish rival by 848, and in the following year, he celebrated his victory by building a church dedicated to St. Columba in his new Pictish lands. The _Scottish Chronicle_, the main source for Scottish History of this period, records that Kenneth raided England no fewer than six times. He died on 858, and was succeeded by his brother Donald I. He had at least four children.

United both peoples through his royal Pictish & Scottish (Irish) blood.9