Armstrong originates in the area along the western Scottish borders; the first recorded bearer was Adam Armstrong, pardoned in Carlisle in 1235 for causing another man's death. They were among the most notorious of the riding Border clans, who also included the Elliots, the Grahams and the Johnstons, famous for their lawlessness and plunder. When the power of these clans was savagely broken after 1603 by James I, the Armstrongs scattered, and many migrated to Ulster, where a large number settled in Co. Fermanagh. Even today, Fermanagh is home to the largest concentration of Armstrong families in Ireland, although the name is quite common throughout Ulster, particularly in counties Antrim and Tyrone.
As well as those of Scottish origin, however, a good number of Irish Armstrongs are of Gaelic Irish extraction. Many of the Trin-Laverys of Co. Antrim and the Trainors of counties Tyrone and Monaghan had their surnames mis-translated as Armstrong, from the presence of the Irish for "strong", trean, in their original names.