I have lots and lots of Johnston threads, on both sides of my family !!
However, Isaac Johnston & his family lived in the city of Armagh on the Mall.  It just so happens that I discovered his parents, William & Rachel Johnston, living on a farm in Middletown, Co Armagh.  That is about 5 miles from Armagh city. In the 1901 & 1911 census, Isaac is living in Armagh city but he has a list of siblings living in Middletown.  
On both census Isaac was born in Co Tyrone and all the others in Middletown, Co ARmagh.  That is why I would like to find out why Isaac was born in Tyrone!!
Liz