"Rev. James McGregor, who emigrated from Londonderry, Ireland"
That's in the North of Ireland,he's descended from Scots on the Forbes side.
He's as Irish as my German Short Haired Pointer.
Don't forget that the Highland Scots (and I believe both McGregor and Forbes are Highland names) are primarily the descendants of Irish invaders in the 6th to 8th Centuries. OTOH, the Lowland Scots are partially Germanic, descendants of Anglians from northern England, mixed with Picts and Britons of Celtic origin. Furthermore, many common names in Ireland, such as Cunningham, Clarke, Burke, Fitzgerald, and Puckett, are of English or Norman French derivation.
Neither the mostly Protestant Scots nor the mostly Catholic Irish can claim anything like purity of blood or a vast distinction from the other ethnic group based on genetic heritage.