Yes this is very true! It has a history far beyond the Enlightenment too, in fact I can't speak specifically to that period but the conception of the Scottish Highlands as an extension of Ireland goes back a long ways. Right back to the medieval and early modern period.

Parts of what became Scotland were colonised by the Irish in the late classic/early medieval period. In fact the word “Scot” comes from Scotii, a Latin term for the Gaelic Irish (or one of the peoples that lived there). Ireland was Scotia Maior (greater Scotia) and Scotland Scotia Minor (lesser Scotia). Although obviously in time ‘Scot’ became confined simply to Scotland, as late as 1005AD we find Brian Boru having himself declared as Imperator Scotorum (generally translated as Emperor of the Irish). The parts of Scotland which were colonised by Gaelic speakers continued with this cultural inheritance for centuries to come. Namely we are talking about parts of the Scottish Highlands and the western Isles.

There existed a common Gaelic World, or Gaedhealtacht which stretched from Kerry to Cape Wrath. The Gaedhil spoke Gaelic (Gaedhilge), but might live in either Alba (Scotland) or Éire (Ireland). This shared world exist beyond the 'borders', as it were, of the Kingdom of Scotland and the various Irish lordships.

This Gaelic world was unified by blood and language; in the belief in a shared origin (the Gaels of Scotland were obviously aware that they had descended from people who came from Ireland) with dynastic and familial ties, along with more mundane things like trade, the movement of mercenaries (galloglasses and redshanks) and “common custom” - ie. similarities in social structure, values and expectations, in formal religious expression and so on. The Gaels of late medieval Ireland and Scotland inhabited very similar mental landscapes.

Most important perhaps was language. Although dialects had been diverging for some time on both sides of the channel, there was a shared literary language - which Scottish scholars refer to as Classical Common Gaelic and which Irish scholars call Early Modern Irish! This was the language of poets and of the learned classes. All of these things - poetry, history, law, music and medicine - was conducted on a formal, organised, professional basis through this language. Thus practitioners of high Gaelic culture were able to move with ease throughout this shared cultural world, between different schools of learning and between different courts. Something which continued right up until the 17th century.

Although not directly unified in political terms (with the Kingdom of Scotland and the patchworks of Irish lordships coming under English colonising), there were dynastic ties and political links. Both regions were highly interconnected through ties of dynastic politics and common political and military interest, particularly north Ulster and the Scottish isles. The most notable case perhaps being the MacDonnell Lords of the Isles, who also penetrated into the Glens of Antrim.

There are numerous examples of political allegiances and intermarriage, as well as recognition of a pan-Gaelic political sphere. For instance, Alexander MacDonald, third lord of the Isles (c. 1423-49) is described in one poem as:

“Pride of the Gaedhil; I The champion of Ulster / . . . The sun of the Gaedhil, / the countenance of O'Colla; / By the banks of the Bann, / quick are his ships; / An angry hound / that checks plunders, / A modest heart, / the tree of Banbha / The land with fire-brands / is red after him; / His prime purpose / is to come to Tara / Putting Meath in commotion, / the leopard of Islay"

Banhba was another contemporary term for Ireland, and Tara was the traditional inauguration site of the High-Kings of Ireland. So although this reflects flattery from the poet more than any actual political reality, it nonetheless points to this shared connection.

Throughout the medieval period that was constant traffic of people and goods across the North Channel through all parts of this shared world. As one anonymous professional poet of the mid-seventeenth century noted:

“The Gael of Scotland and of Ireland long ago were the same in origin and in blood, as our schools relate.”

And indeed, those in the lowlands of Scotland also recognised this shared culture. Speakers of English or Scots consistently described the Gaedhil as 'Irish', or 'Erse' (a variant), regardless of whether they lived in Scotland or Ireland.Something which clearly reflects a continuing perception of the primary identification of that people and culture with Ireland.

In 1542, just before the battle of Solway Moss, an English spy reported of Argyll's troops in the Scottish army, which was then approaching the English borders:

“that therle of Argile had with him…[12,000] Yerishe men and…and that the Scottis were more aferde of the saide Yrishe men than of thEnglishe army, for they did asmuche hurt where they cam in distroying the corn and vitailles and not paying therfore, and if they were resisted by the saide Scottis they wolde kill them."

This spy then provided a description of the customs and 'maner of the said Yrishe men' which was fairly unremarkable, apart from the fact that most of these supposed Irishmen would actually never have set foot in Ireland. Both the English state and the (lowland) Scottish state held similar views of it’s Gaelic population - as a ‘barbarous’ people who would need to be tamed and brought into civil society, and thereby integrated into their respective Kingdoms.

Not to say there were no changes across this period, of course it is messy like anything in history, but there is nonetheless a remarkable degree of persistence across this unified cultural sphere right up until the seventeenth century.

I’m not at my computer right now, but if you can wait until tomorrow I’ll happily answer any follow-ups, provide more detail and some suggestions for further-reading.

'Picts Gaels and Scots' by Sally M Foster (2014 ed) is a good introduction to early medieval Scotland including the origins of Dál Riata. It has been on reading lists at universities in Scotland for years.

Also, the work of Tim Clarkson is helpful about Strathclyde and neighbours in same era. 'Strathclyde and the Anglo-Saxons in the Viking Age' being one example work.

In terms of books about Scotland and Scottish identity, you might be interested in David Ditchburn's Scotland and Europe: The Medieval Kingdom and its contacts with Christendom (2000), which explores the very close relationship Scotland had with northern Europe.

Dr. Ewan Cameron of Edinburgh University has done a fair amount of work on Glasgow and the Clydeside in the 19th and 20th centuries. Some of them are open access, like his 2004 article "‘Alas, Skyemen are imitating the Irish’: A note on Alexander Nicolson's ‘Little Leaflet’ concerning the Crofters' Agitation."

For the British Isles more generally, I believe the classic text is Linda Colley's Britons: Forging the Nation (1992) although I confess I have not read it.