I have a little on JOHNSTONS near Clones in Co. Monaghan and in neighboring counties Fermanagh and Tyrone, and will be happy to share if you're still researching this family.
JOHNSTONS are mentioned in various historical records of Clones parish and County Monaghan, and if you contact local historian and author Theo McMahon in Monaghan town he may be able to tell you more (if he's not innundated with queries, which is usually the case!). Also in southern Tyrone, there's researcher and author Jack Johnston, who edits the multiple-county historical journal, "The Spark."
Back issues of the "Clogher Record" genealogical journal have great data on early Co. Monaghan families as well as surveys of some early cemeteries -- St. Tiernach's (Tighernach's) Church of Ireland in Clones has JOHNSTON burials, for instance.
JOHNSTONS are also found in records of Stonebridge Presbyterian Church near Clones, including marriages of my relatives ELIZA JANE CALVIN & WILLIAM JOHNSTON in 1837, and Eliza Jane's sister MARIA CALVIN & MARTIN JOHNSTON in 1845.