Cheekywan,
You got it right.
It seems before David and Margaret got married, the "Irishman" and Margaret were quite close as she got pregnant by him.
I think it may have been the case of Irish eyes a smiling.

Donald Grant Smart is actually the son of the "Irishman" and Margaret, the rest of the children are from David and Margaret.
It was no secret to the family, it had been discussed on a couple of occasions but as with all things what does appears not to be important at the time is not remembered and the sad thing is that those who were in the "know" have all since passed away.
In tracing our linage, it would only be proper to do a complete job of it.
What I find unique about Donald's first name is that there were no Donalds in the Smart's line nor any in the Grant's line (that I have found so far).
This kind shoots the paternal or materanal nameing sequence in the foot.