The pre-plantation Ulster Irish are as close as you get in Ireland to the Highland Scottish society. The Ulster Irish and clans dwelt in the area of present day Northern Ireland, and were regarded as being a sort of backwater and very conservative region and population in Ireland. The Ulster dialect is also closest to the Scottish Gaelic tongue [1], and the Scottish Gaels themselves have their origins from this region wherein they diffused and spread out from western Scotland and highlands and throughout the present day country.

To better explain the strong ties between Ulster Ireland and Highland Scotland - and Scotland in general - I'll give a brief outline of the ancient connections and history between the two respective regions.

It all began with the migration of Irish from Ulster - historically and anciently named 'Ulaid' - to the western seaboard of present-day Scotland. These Ulaid Irish - ancestral Ulster Irish - began migrations and settlement of the western Scottish seaboard during the 1st half of the 1st millennium BCE. The centre of settlement of the Ulaid Irish settlers is ascertained to be present-day Argyll, Scotland [2].

During this early period, present-day Scotland was largely inhabited by a Brythonic Celtic people referred to as the Picts. The Picts were mostly related to in culture, language, and of same origin with the other Brythonic Celts such as the the ancestors of the present-day Welsh and Cornish, and other Britons [3] [4].

Prior to the Ulaid Irish migrations, all of Britain was inhabited by Brythonic Celts, one of the major branches of the Celtic peoples alongside the Gaelic Celts. Scotland itself was mostly Pictish, with other non-Pictish Brythonic Celts probably inhabiting the more southerly reaches, and perhaps, most of the lowlands as well.

As such, the Ulaid Irish settlers were amongst the first Gaelic Celts present in Britain. The Ulaid Irish settlers in western Scotland also brought with them their Old Irish tongue, and their Scottish settlements in Argyll and western Scotland formed a westernmost extension of the Ulaid Irish subkingdom of Dal Raida, making Dal Raida and the larger and encompassing Kingdom of Ulaid a kingdoms of two Isles encompassing Dal Raida's holdings of County Antrim in northern Ireland (Ulaid), and Argyllshire in western Scotland [6].

Historically and traditionally up until the plantation period, the Ulaid Irish and their descendant Ulster Irish were defined by their adherence to a significant degree of pastoralism and transhumance [7]. This pastoralist tradition persisted up until the plantation period, and formed an integral part of Ulaid and later Ulster Irish life [7]. The importance of cattle in early Ulaid Irish and Irish society and the subsequent settlement of western Scotland by the earlier Ulaid Irish may have led to a spread and/or resurgence of pastoral transhumance in the Highlands which may have not been as present in the Brythonic Pictish period.

After initial Ulaid Irish settlement of Argyll and the western Scottish archipelago and coast, strong seaborne cultural and ethnic connections were maintained by extensive seaborne contact. The Ulaid Irish settlers and the kingdom of Dal Raida had begun to become dominant and exude power from their bastion in western Scotland. The Old Irish brought by the Ulaid Irish settlers had also started to diverge from Irish as the descendants of the Ulaid Irish, the Scottish Gaels, began assimilating the Brythonic Celtic Picts. The merger of Picts and Scottish Gaelic peoples as the Gaels expanded from the western seaboard and into the Highlands and lowlands and eastward continued up until the 12th CE, by which time the Picts were like to have been completely forgotten and fully assimilated by the Scottish Gaels.

The assimilation of large numbers of Brythonic Celts - mostly Picts - by Scottish Gaels resulted in a heavy shift in the grammatical syntax and verbiage of the Old Irish-descended Scottish Gaelic from its Gaelic origins and increasingly came to towards resembling the Brythonic Celtic languages in many ways [8].

Later on the Kingdom of Alba is formed with a final merger of Gaelicized Pictish aristocracy and Gaelic Scots, and present-day Scotland is dominated by Gaelic-speakers. Sustained contact between Ulster Ireland and the Kingdom of Alba (Scotland) continues. Marriages between notables from both shores were somewhat common.

Another very important set of points I should have raised earlier is the clans and kinship ties between the Ulster/Ulaid Irish clans and the Scottish Highland clans. The most important point being how genealogies of many Highland clans claim origins from Ulaid Irish clans and other non-Ulaid Irish persons, such as Niall of the nine hostages via the O'Naills, or Ulaid ones like the Dal Raidan monarchs. Such connections likely served to strengthen the sense of kinship between the Ulster/other Irish and Highland Scots [9].

Like the Highland clans, the Ulster Irish were also very resistant to the English in comparison to their respective less-resistant countrymen [10], and both groups were inhabiting regions considered as economically lacking, less orderly, 'uncivilized', and empty [11]. Also, as happened to the Highland Scots with the Highland clearances, the Ulster Irish and their territory was targeted by the English due to their respective reputations for resistance and anti-English sentiment as well. Ulster territory was ground zero for the 'plantation' and settlement of English and Scottish protestants in Ireland, and the Ulster Irish had to be militarily subdued. The influx of protestants, as well as the allocation of Ulster Irish land to these settlers led to the displacement of the Ulster Irish, who were reduced to becoming tenants in their own land and dispossed [11]. The Ulster Irish territory became anglicized to the point where the traditional Irish tongue and especially the Ulster dialect is rarely spoken in many parts of Northern Ireland. Also a result of the Ulster plantation, protestants came to form a large portion of the population as well as give a strong British and unionist identity to present day Northern Ireland.

Also interesting to mention, the first 'Scoti' or 'Scots' recorded in Britain were the Ulaid Irish settlers. The term was also used to refer to all Irish, and later on to all Gaels. At some point it came to exclusively refer to the Scottish people as of today [12].

[1] Borsley, Robert D., and Ian G. Roberts. The Syntax of the Celtic Languages: A Comparative Perspective. Cambridge: Cambridge UP, 1996.

[2] Ellis, Peter Berresford. The Celtic Revolution: A Study in Anti-imperialism. Talybont: Y Lolfa, 1985.

[3] Stafford, Pauline. A Companion to the Early Middle Ages: Britain and Ireland C.500-1100. Hoboken, NJ: Wiley-Blackwell, 2012.

[4] Lynch, Michael. The Oxford Companion to Scottish History. Oxford: Oxford U, 2011.

[5]

[6] Halpin, Andrew, and Conor Newman. Ireland: An Oxford Archaeological Guide to Sites from Earliest times to AD 1600. Oxford: Oxford UP, 2006.

[7] W., Dowling Martin. Tenant Right: Agrarian Capitalism and Traditional Agriculture in Rural Ulster, 1600-1850, 1994.

[8] Thomson, Derick S. The Companion to Gaelic Scotland. Oxford: Blackwell, 1983.

[9] Thornton, David E. Kings, Chronologies, and Genealogies: Studies in the Political History of Early Medieval Ireland and Wales. Oxford: Unit for Prosopograohical Research, Linacre College, 2003.

[10] Biagini, Emilio. Northern Ireland and Beyond: Social and Geographical Issues. Dordrecht, the Netherlands: Kluwer Academic, 1996.

[11] Kennedy, Liam, and Philip Ollerenshaw. Ulster since 1600: Politics, Economy, and Society. Oxford: Oxford UP, 2013.

[12] Ball, Martin J., and James Fife. The Celtic Languages. London: New York, 1993.