The only reason that Lothian was, in time, to become the primary metropolitan county of Scotland - rather than a remote Northern province of England - was due to its absorption into the Gàidhealach (Gaelic) kingdom of Alba. "Alba" derives from the ancient name of Northern Scotland, the first historical inhabitants of whom were the Picts. But around the middle of the first millennium, a Northern Irish tribe known as the Scots (Scotti, in Latin) began to settle in Argyll on the Midwest coast, just North of the Brythonic kingdom of Strathclyde. By the end of the first millennium, their kingdom of Dál nAraide (Dal Riada) had separated from Ulster, absorbed Pictavia, and began making further inroads South of the Rivers Forth and Clyde. Gàidhlig thus began to filter into the Lothians through aristocratic settlement, and many placenames, even within Edinburgh itself, reveal Gàidhlig roots (such as Drumbrae, from An Druim Bràighe, or Dalry, from Dail an Rìgh).
Following the existing placename evidence, it seems that the heaviest settlement areas of ordinary Gàidhlig-speakers (rather than necessarily just Gàidhlig-speaking nobles) was in the Western areas of West Lothian, Edinburgh itself, and the Northern foothills of the Pentlands around the area of Currie and Balerno. However, Gàidhlig never became the language of the Lothians - indeed, the Germanic language of the region, Scots, gradually became the primary political, and eventually cultural, language of the majority of Scotland (at least in the South and East).
Nonetheless, in modern times, the populist history of Scotland has often over-invested the nation's Celticity in Goidelic, rather than Brythonic concepts, such that even many modern inhabitants of Lothian labour under the illusion that "before the English came", the region was as Gaelic as the Highlands. No doubt the populist myth has much to do with the popularisation of often equally fake images of the Highlands from the time of Robert Burns and Walter Scott onwards (both Lowlanders, of course), and the creation of the stereotypical tartanry with which twentieth century tourist policy tended to sell Scotland. But it seems that even in the medieval period, serious Lowland historians found it hard to understand their deep Celtic heritage as fully Brythonic, deriving it instead from some notional ur-Scotland from the North. As a result, we find medieval histories of Lothian claiming that the first historical inhabitants were Picts and not straightforward Britons.
Today, though, a whole range of disciplines from literary history through archaeology has shown us that South-east Scotland was barely touched by any large-scale or lasting Gaelic cultural influences, or even any significant influx of population. But at the same time, if Gaelic Alba had not incorporated the region into its own structure as a Southern border zone between that Northern kingdom and Anglian Northumbria, then perhaps modern Lothian would not be Scottish at all.
The significant event in Lothian's Gaelic history comes in 1018 at the Battle of Carham, a battle fought between the two major rival kingdoms on either side of Lothian - Alba and Northumbria. If the Battle of Dunnichen in 685 ensured that Northumbrian expansion stopped at Lothian and basically left Pictavia alone, then the Battle of Carham formally pushed Northumbria back to its yet earlier Northern borders, and secured Lothian as a Scottish territory, back again in Celtic hands, if only on the political level. More globally, Carham was in some (perhaps simplistic) senses, the event which defined the modern borders between Scotland and England.
more to follow...