The Scots (from the Roman term Scoti), who invaded and settled in Argyleshire (possibly from the area around County Antrim in what is now Northern Ireland) at the end of the fifth century, and ultimately gave their name to the whole of North Britain, seem for a long period to have confined themselves to their little settlement of Dalriada, in the West Highlands, and to have made no endeavour to enlarge their territory.
The Picts governed the remainder of Alba and had their capital and royal residences at Abernethy and Forteviot in Strathearn. Down to the middle of the eighth century the two dynasties seem to have reigned together over their respective territories (that of the Picts being much the larger) without any serious attempts at dispossession on the part of either. About the period last named, however, the Scots were completely subjugated by the Picts, who for nearly a hundred years remained masters both of Alba and Dalriada.
Then a Scots prince, named Alpin, laid claim to the Pictish throne, but was overthrown and put to death by the Pictish King Drust (or Drest) the tenth. Alpin's son Kenneth resolved to avenge his father's death, and to reassert his claim to the crown. As a part of his campaign, local folklore states that he gathered his troops on Baingle Brae in Tullibody, in preparation for a decisive battle at Logie (east of Airthrey Castle), near Stirling. During this battle he routed and scattered the Pictish King's forces. Later King Drust was killed at Scone - the Pictish capital - and eventually Kenneth MacAlpin established himself as sole king of the Picts and Scots. Standing stones were placed after the battle at both the battle site near Stirling University, and at Tullibody. This stone said to have been broken up during a realignment of the road from Tullibody to Stirling in the early 20th century.
King Kenneth went on to conquer other areas of what eventually became the lands of modern Scotland, a process that was taken up by the succeeding kings. In the seventh year of his reign, he is said to have transferred part of the relics of St Columba to a church which he had built—an incident which seems to mark the transference of the ecclesiastical metropolis from lona to Abernethy on the south bank of the Tay.
The name of the Picts gradually disappears after this from history, and the Scots are the rulers of Alba, or the country to the north of the Forth. It is not till nearly two hundred years afterwards, under Malcolm II, a descendant of Kenneth, that we find Alba, Alban, or Albany, coexistent with Scotland, as we now understand the term. This increased extent of sovereignty arose in consequence of the incorporation with the realm of Alba, partly by transfer, partly by conquest, of the British kingdom of Strathclyde, and the portion of the Anglo-Saxon kingdom of Northumbria lying to the north of the Tweed.
References:
Between the Ochils and the Forth - David Beveridge (1888)
Wikipedia
Britannica.com