The marriage will have been a Catholic one and registered at St Michael's Church, Ardkenneth though it will have taken place in Uachdar, Mr Seumas [Fr James MacGregor] officiating. Some but not all such records seem to have found their way onto the IGI. The only certain way to see the relevant records is, I am afraid, to go to Ardkenneth and see the leather bound originals in person. On the assumption that is not possible for you, somewhere I do have a transcript which I will try to track down in the mass of paper in my studio.
As for Catriona Michael, her sister Ann married Angus MacMillan from Balivanich, a different MacMillan family and they emigrated to settle at Grand Lake, CB. Their father, Michael MacDonald and family also emigrated and are recorded in one of the two Fr MacMillan books, from memory 'A West Wind to East Bay'.
I would be interested in what happened to the descendants of John. Are there any males still with the MacMillan name? I have always hoped some might show up and be tempted to do a Y-DNA test [ a simple cheek swab] as the records frustratingly just fail to go far enough back to allow the Uachdar, Balivanich and my Torlum MacMillans to be sorted out; related or not related?