In the 1840s and 1850s the population was swelled by a great influx of Irish from all the provinces of that country. This was the first great movement of people between the two countries, and it brought back to Scotland many of the descendants of the Plantation of the early 1600s. It also brought into Scotland substantial Catholic population, who were to have an impact on the political scene as well on the religious make up of the country. Among the Irish were many who were simply taking refuge from desperate poverty and famine. They settled primarily in the Glasgow area since their was work available there and their predecessors had already established bridgeheads of settlement. By 1841 the Irish made up 16 per cent of the population in Glasgow. The Irish, but whose own union with England in 1801 was more akin to a colonial relationship then the negotiated settlement between Scotland and England of 1707, were no respecters of the English and brought with them a tradition of demand for Home Rule and the willingness to use violence against what they regarded as a government sustained by military force and buy an imported oligarchy of the wealthy.
An inevitable part of their baggage was the entrenched antipathy nourished in Ulster between pro-Union Protestants and anti-Union Catholics. The two Irish factions did not intermingle, and the arrival of so many Catholics created some alarm within leading Presbyterian circles and something of a Protestant backlash. Steps taken in the 19th century towards national and municipal order to place against a constant background of official and public fear of violence: the violence of militant Irish nationalism, of revolutionary anarchism and even the potential violence of Scottish working men who were now employed in army strength even in single factories, whose minds might inflamed by agitators and who showed a disturbing tendency to organise themselves into united groups in order to put pressure on the employers.
On the other hand, Scottish immigration has played a prominent role in the development of Ireland. Edward Bruce, the brother of the famed Scottish king Robert Bruce, invaded Ireland in 1315 and nearly established a unified Scottish-Irish state at the expense of the Anglo-Norman Plantagenets. Scottish mercenaries (Gallowglasses) were the backbone of the military structures maintained by many northern Irish lords throughout the late mediaeval and early modern periods. Ultimately, the greatest Scottish impact on Ireland came in the form of long term migration during the 17th century. This heavy Influx of Scottish, largerly Presbyterians, settlers in Ulster radically altered the religious composition of the northern province.
The notorious, officially sanctioned Ulster Plantation (1609 – 25) displaced many native Irish landholders, but peaceful Scottish immigration had both preceded and continued long after these dates. In counties Antrim and Down, Scots came to constitute the population’s absolute majority. There were also major settlements in Derry, Armagh, Tyron, and Donegal. Although Presbyterians were at the forefront of secular Republican nationalism in the 1790s, by the mid-to late 19th century, they had largely become loyalists. In the present day, the descendants of Scottish settlers comprise the majority of the northern Irish Protestant community.