Until a few years ago, I thought my Stuart ancestor was the only family member to leave Ireland and settle in Canada. We now believe (we cannot yet confirm it) that we found his parents and at least 3 brothers and 2 sisters living in his (Canadian) town and neighbouring towns. I wonder if your McDonald/McGinn couple's siblings could have also settled in North America?
There is a bit of a glitch, but I wonder if there could be a connection...
1880 Boulder census
James Mc Donnald, age 47, Tailor, born Scotland as were parents. His children are also listed.
Shown on the same page:
John R(?) McDonald, age 38, Huckster(?), born Scotland, father born Scotland, mother
born New York 
Susan McDonald, age 39, wife, Keeping House, born Maine as were parents
Also in this household:
Geo W McDonald, age 20, brother, Express Waggon, born Ohio, father born Scotland, mother born New York
To see how many McDon*/Mc Don* (born in Scotland) were living in Boulder in 1880, I tried a search and the only "hits" were for the two men. Was it just a coincidence that (if the search truly produced the correct results) only
two McDon*/Mc Don* (born in Scotland) were living in Boulder and were shown on the same page or could the men have been related? It might be worth researching the other family to see if there is a connection.
If you haven't already done so, and you are fairly certain that Susan's surname was McGinn, it might be worthwhile to see if any of her family members came to the states (by searching McGinn, born Ireland c1838, etc.).
Was it somewhat common to see the McDonnald spelling? Are you certain their name was McDonald and not McDonnell?
Just something to think about.
