Interesting discussion. My Stewart family has roots in Ramelton going back to about 1800, when a Thomas Stewart was listed owning the Roughan, my grandfather's farm just outside Ramelton. I have not, however, been able to link my Stewart ancestors back to any of the Ulster planters, so I just don't know where these Stewarts might have sprung from.
On my mother's side (also from Donegal) I have Babingtons in the early 1800s back to Brutus Babington, Bishop of Derry and beyond.