A new missive from Scottish researcher Duncan McGruther:

I have recently confirmed some information that was not available to me when I wrote my book MacGrouthers in Scotland to 1855, and feel I am in a position to make some reasoned speculation on the source of the McGruthers of Meigor wealth. In 1620 the McGruder family, tenants in Innerclair alias Wester Craigneich, acquired the lands of Meigor near Comrie, Perthshire. In an era when it was almost unheard of for any tenants to acquire land, they became the first McGruther landowners in Scotland. The date is intriguing. Around this time the Dukes of Perth family became one of the ‘Undertakers’ in what is now Northern Ireland of a scheme to populate the land there with Scottish and English tenants – the Plantation of Ulster. Scottish Perthshire names begin appearing from then on in Ulster, including John McCrue, father and son, whose ancestors’ written testimony speculates their name was derived from McGruder.
Thus, the Drummonds of Perth, to curry favour with the King (whose Scheme was the Plantation of Ulster) undertook to supply tenants for Ireland. The McGruders supplied their junior family members, but extracted Meigor in Scotland as their price. The McGruders who moved to Northern Ireland themselves got 9000 acres of land. The Source of the McGruder/McCrue name is thus Perthshire, and there is no separate Irish McGruder (or variations thereof) name.
This may explain why, since [my] book was published in 2007, no Ulster McGruder/Magruder/etc has come forward.