Scotland to Ireland, the First Migrations

The migration of Protestant Scots to Ulster, Dublin, and other parts of Ireland, occurred mainly during the 17th and 18th centuries. The first major influx of Scots into Ulster came during the settlement of east Down. This started in May 1606 and was followed in 1610 by the arrival of many more Scots as part of the Plantation of Ulster. During the Irish Rebellion of 1641, Irish Catholics attempted to expel the settlers, resulting in inter-communal violence and, ultimately, leading to the death of between 10,000 and 20,000 settlers and an undetermined number of Irish people, over 10 years of war. The memory of this traumatic episode, and the savage repression which followed, poisoned the relationship between the Scottish and English settlers and Irish Roman Catholics, almost irreparably.
The Scots-Irish population in Ulster was further augmented during the subsequent Irish Confederate Wars, when a Scottish Covenanter army was landed in the province to protect the settlers from Irish Catholic forces. After the Wars were over, many of the soldiers settled permanently in Ulster.
Finally, another major influx of Scots into northern Ireland occurred in the 1690s, when tens of thousands of people fled a famine in Scotland to come to Ulster.
The settlers and their descendants, the majority of whom were Presbyterian or Anglican, became the majority in the province of Ulster. However, along with Roman Catholics, Presbyterians and other non-Anglican Protestants were legally disenfranchised by the Penal Laws, which gave full rights only to Anglicans, who were mainly the descendants of English settlers belonging to the Church of Ireland. For this reason, up until the 19th century, and despite their common fear of the dispossessed Catholics, there was considerable disharmony between the Presbyterian and Anglican populations of Ulster. Subsequently, in 1798, many Ulster-Scots joined the United Irishmen and participated in the Irish Rebellion of 1798.

(Source: Wikipedia, http://en.wikipedia.org/wiki/Scots_Irish)