The history of the Gaels and their languages (4 are left) is a rough one. While we will mainly deal with the Scots Gaidhlig, we definately have to primer the other three.
The Early Gaelic Language
It is incorrect to think of Scotland as a wholly Celtic country. Since the first millennium BC, Scotland has been a place of multiple languages and this tradition continues today. First of all it was Pictish and Brythonic; then Gaelic, Norse and Scots came and today it's English, Scots and Gaelic. Nearly all of Scotland was once Gaelic speaking except Orkney, Shetland and Caithness which had a variety of Norse until recent times and East Lothian which was settled by the Angles. Galloway had a Gaelic community which became separated from the Gaelic speaking Highlands and Gaelic was still in use until about the 17th century in Galloway. Gaelic is a Celtic language, like Irish, while Scots is a Germanic language like English. You know how we think of the Scottish people as attempting to speak a language foreign to them (English) and their Gaelic accent kicks in? Well, that's remarkably incorrect. "Scots" is it's own language, which grew the same time as English did and they both derived from Germanic origins. The Scottish people who spoke Scots lived near and among the English so long the two sort of infused. What we think of as today's "Scottish accent" is actually a more understandable form of the old Scots language.
Scottish Gaelic
In Britain (including Scotland), Brythonic Celtic predates Gaelic by almost 1000 years or so. Being spoken from Kent up to Glasgow and across to Wales. Some people even suggest that Brythonic was spoken in Ireland before Gaelic, but this notion begs the question... "Where did Gaelic come from and when?" But that's another story. Pictish (possibly a Celtic language) would probably predate even Brythonic.
(From Scottish Gaelic in Three Months, by Roibeard O Maolalaigh and Iain MacAonghuis, Hugo Language Books
Ltd., 1996)
Gaelic was brought to Scotland by colonists [ed. refugees] from Ireland towards
the end of the Roman Empire in Britain. By 500 A.D. these Gaels had established their Kingdom of Dal Riada, centered
on what is now Argyll in southwest Scotland; in Gaelic, Earra Ghaidheal, "the coastland of the Gael." To Roman
writers they were Scotti -- Scotia at this time denoted Ireland [ed. the Latin name "Scotia" was no longer used
to mean Ireland, but the kingdom we now know in English as Scotland. The war cry of Scottish soldiers, used in war
against the Vikings in 903, against the Danes of York in 918 and against the English in 1138, was Albanaigh,
Albanaigh] -- although these names cannot be traced with certainty to an origin in Gaelic itself. But from these
Latin forms came the name Scotland. In Gaelic, however, the country is Alba, as in Irish Gaelic, and Alban in
Welsh. [ed. It is important to realise that the modern national geographical boundaries of Ireland and Scotland
did not exist; only a narrow channel of water. Gaels secured a lasting success in Scotland when Dal Riada under
Kenneth Mac Alpine, became merged with the Pictish kingdom of Fortrui, which had already consolidated Pictish
authority to a large degree.]
Pictish: The Picts were Celts but spoke a mixture of languages. They spoke a pre-Celtic language for ritualistic purposes (source: Prof Derek Thompson - "Why Gaelic matters"), and Pictish at other times. Pictish is mentioned in The Cambridge Encyclopedia of Language as possibly being Celtic or possibly being a non-Indo-European isolate like Basque. Thompson says "It is clear from the evidence of place names that there was much common ground between [Brythonic] and the Celtic constituent of Pictish". There is some debate as to whether Pictish was non Indo-European or not, as there is so little information available on it.
Many of the Scottish Island names including Arran, Skye, Lewis and Jura are Pictish. For more information on placenames: (W.F.H. Nicolaisen "Scottish Place Names", Batsford, London 1976).
As to Gaelic and English in Scotland, The Highlands of Scotland were occupied by Picts and the Lowlands were occupied by Brythonic Celts. The Romans occupying the Lowlands during this time and when the Romans left in 407, they left a weak kingdom, but still brythonic. The Scots (Gaelic speaking) extended their region of Dalriada into Argyllshire, between 500 and 550. The Angles extended the Kingdom of Northumbria into Lothian, Berwick, Selkirk, Peebles and Roxburgh. As far as I am aware these areas are in present day Scotland. The Angles spoke a dialect of what is know today as "Old English". The Angles moved into this area about 540 -600, these are rough dates. As time went on, Scotland was left with 4 distinct areas. Dalriada, Pictland, Strathclyde and Lothian (Northumbria). In 625 the Northumbrian Kingdom stretched from the Humber to the Forth and was ruled by Edwin. In 685 the Northumbrians decided to try and extend Northumbria into Pictland and hence invaded the Picts, but this was a big mistake. The Northumbrian army was defeated by the Picts and eventually Northumbria lost supremacy to the Southern Saxons. (Also why RP is based on Southern English and Not Northumbrian ???). The Picts became the supreme overlords of the Scots in Dalriada and the Brythonic Celts in Strathclyde.
About 785, Pictland started to receive attacks from bands of Norse invaders and hese lead to Pictish defeats and in the 830 (approx), the Norse invaders made permanent settlements.
In 843 Dalriada threw off Pictish control, where upon the Scots King Kenneth MacAlpine laid claim to the Pictish throne through the Celtic law of Tanistry. Followed by the union of the Picts and the Scots. The now "United Kingdom" tried to oust the Northumbrians from Lothian but were unsuccessful. At this time the Norse people occupied the Western Isles, Northern Isles and Caithness.
The Scots allied themselves to the English to get rid of the Norse Invaders and sometimes allied themselves with the Norse to get rid of the English.
It was not until 1018 that the Scots Kingdom managed to remove Lothian from the hands of the Northumbrians and in 1034 the Scots, Angles, British and Picts were a United Kingdom of Scotland.
As far as I am aware MacBeth was the last of the Gaelic Kings, and he himself was followed by Malcolm, whose wife (an English lass) moved the royal court to Edinburgh around about 1070. At this time many persecuted English people moved into Lothian from England due to Norman Conquest. The English who were persecuted in England flourished in Scotland.
The real point of all the above is that English has been spoken since the 6th Century in Scotland. Not all of it but quite a large piece. The modern Scots language dates back to the first Angle invasions at this time. Incidentally whilst parts of Scotland were English speaking, parts of England were still Celtic speaking {eg; West Yorkshire Kingdom of Elmet and part of Cumbria.) To sum up English has been spoken for longer in Edinburgh than in Leeds.
By the eleventh century [ed. The Dark Ages], Gaelic was at its highest point in Scotland and known to some degree virtually throughout the country. A Gaelic-speaking court, supported by the Columban church, gave patronage to makers of literature at the highest levels of society. With the Anglicisation of the dynasty late in that century, what was been described as a shift to an English way of life was deliberately planned and, as far as possible, implemented. The court itself became English and Norman-French in Speech and the northern English dialect (Inglis) was fostered as the official language. The loss of status that these changes entailed for Gaelic had a profound and permanent effect. In the mid-twelfth century the Lordship of the Isles, founded in part on the Norse kingdom of the Western and Southern Isles[ed. Norway officially returned control of the Hebrides to the Scottish crown in 1266.], but drawing also on the traditions of a former, wider Gaelic territory, emerged as a quasi-independent state. Until the Lordship was destroyed by the central authorities of Scotland in the late fifteenth century, Gaelic culture and learning continued to flourish. In the same twelfth century a reorganized literary order, whose main centers were in Ireland, was codifying Gaelic to produce an elegant formal register of the language, which we call Classical Gaelic. It was common to the learned classes of Ireland and Scotland and taught to the children of the aristocracy. It lasted in Scotland until the eighteenth century.
While it is true that the history of the language is largely one of resistance to ethnocidal policies that sought to exclude the Gaels from the world of post-Renaissance Europe, contemporary developments in education, radio and television, and in literature generally, aim to redress the balance. And it should be noted that some of the most interesting writers now active on the literary scene are not native speakers but learners of Gaelic.
The eighteenth and nineteenth centuries saw large numbers of Gaelic speakers removed from their land in the Highlands and Islands by landlords. Many were forced to emigrate and Gaelic communities were established in parts of Nova Scotia (Alba Nuadh). There is still a considerable number of Gaelic speakers in Cape Breton Island (Eilean Cheap Breatainn), but the language is largely confined to the older generation. Enforced exile has given way this century to voluntary emigration, and expatriate Gaelic-speaking Scots are all over the world, but especially in Canada and the United States (Na Staitean Aonaichte). Many Gaels have also moved south to England for employment.
The Celtic Languages
(From Dictionary of Celtic Mythology by James MacKillop, Oxford Univ. Press, 1998)The Celtic languages: a subfamily of the Indo-European Family of languages. In antiquity, speakers of Celtic languages could be found in what is today Turkey (the Galatians of the Apostle, Paul's, letters), the Balkans, and most of central and western Europe from the Danube valley to the British Isles, including large portions of northern Italy and the Iberian peninsula. In modern times the living Celtic languages have been Irish and Scottish Gaelic, and Manx (now extinct) from the Goidelic or Q Celtic branch, and Welsh, Cornish (now extinct), and Breton from the Brythonic, Cymric, or P Celtic branch. Despite stereotypes, the Celtic languages have been spoken by a wide variety of physical types, from short and dark to tall and fair. Much controversy surrounds speculation on the meaning of the word Celtic. It appears to be derived from the Greek, keltoi, used to denote people a people north of what is now Marseilles. Julius Caesar also reported that the Gauls described themselves as Celtae. The perception that the Celtic languages were all related was slow in coming, and thus the word "Celtic" did not always denote all the Celtic-speaking peoples. Classical commentators did not call the inhabitants of the British Isles Celts, and the word "Celt" has no counterpart in Old Irish or Old Welsh, as speakers of those languages did not see themselves as forming a linguistic community. The term "Celtic language" was not used in English with its present meaning until the beginning of the eighteenth century.