I am researching these same MacIsaacs for my cousin in Scotland (Allan MacDonald of Glenuig, a world-famous piper), but apparently his family were MacIsaacs at some point in the 1800s. One of them went into a fencibles regiment and the fellow who was registering him didn't understand Gaelic, so he put his name down as MacDonald instead. Allan does have MacDonalds in his genealogy too, and MacPherson. I have quite a bit of information from the Scotland side that I am happy to share with all you MacIsaac researchers, in return for what you have on the Giant's Lake MacIsaac. Please contact me off line at firstname.lastname@example.org
and I will give you lots of info.
Marlene MacDonald Cheng
Victoria BC (Born and raised in Antigonish)