Another good factual account is “The Border Reivers” by Godfrey Watson (published 1974). There’s pages and pages of information about the Armstrongs. When discussing the relocation of many reivers to Ireland, Watson says (p195):
“Next on the list were the Armstrongs, and particularly those of Whithaugh. In 1609 a plan was hatched for the voluntary settlement of Ulster in order to reinforce the Protestant population, and great numbers of Borderers, feeling that there was no longer any future for them at home, departed to try their luck in Ireland. Many remained there to become honest, hard working citizens, and particularly to make their mark as soldiers. Others drifted back only to find, in the case of the Armstrongs, that the Scotts and Elliotts had been awarded their lands.”
The surname Armstrong was completely unknown in Ireland prior to this, so if your ancestors were named Armstrong and lived in Fermanagh then it’s 100% certain they were incomers, and probably from the Borders.
There are no records, either in Scotland or Ireland, of the individuals who came but there’s obviously a possibility yours came from Whithaugh (which is near Newcastleton, in Roxburghshire). The ruins of the Armstrongs castle there are open to the public:
http://www.stravaiging.com/history/castle/whithaugh-tower
http://www.welters-worldwide.com/Manifesto/Heritage/Heritage2.htm
The other thing that one Armstrong left us is a very fine song. Originally called “Armstrong’s Goodnight” it was supposedly written by Sandy Armstrong on the eve of his execution (November 1600) for reiving. Until Robert Burns wrote Auld Lang Syne, it was reportedly the most popular song in Scotland.
This night is my departing night,
For here nae longer maun I stay
There’s neither friend nor foe o’ mine
But wishes me away.
What I hae done through lack o’ wit
I never, never can recall
I hope ye’re a’ my friends as yet,
Goodnight and joy be wi’ ye all.
Today the song is more widely known as The Parting Glass. Sung here by the late Ronnie Drew of the Dubliners:
https://www.youtube.com/watch?v=2M3wr4hYSf4