In reading some other threads related to this Alexander Alex, I realise that there is an error in my tree, and most likely, he is not my ancestor.

My third great grandmother, Elizabeth Grant married Garret O'Brien, and I believe the question is who her parents are. On other trees they are Alexander Grant and Sarah Claridge, but they show Alexander as being son of John Duldreggan Grant -- and that seems to be incorrect. Alexander Alex married Nancy McDonell, so something is off.

Thank you for providing so much information. It has helped me in an unexpected way. :-)