1./7.: Mary Cloak's mother, Mary Power Cloak, married William Hogan in the parish of Wexford -- the same parish that Mary Cloak married Martin Kirwan in 1848 -- so you make a very good point.
2.: I have looked for Martin Kirwan in Griffith's Valuation and need to do this again when AskAbout Ireland is next up and responding. As I recall from much earlier searches, there were several Martin Kirwan and there was no way of knowing if any of these records were for my Martin.
3.: Yes, that could be although my understanding is that godparents were traditionally siblings or siblings-in-law. I will have to look at the baptism records for the two Francis' and Patrick again and follow up on this lead.
4.: There were several churches in Wexford Town at various times. Unfortunately, the parish records at the NLI do not indicate anything more than "Wexford, Diocese of Ferns."
5.: The only information that had come down to us is that Martin Kirwan came from Wexford -- no mention of the county vs town let alone a street name or townland.
6.: According to a cousin, Thaddeus is not related to my line. Nevertheless, it would be interesting to learn more about him at some point. Interestingly, based on an interment record from Our Lady of Mercy Church in Sarnia, Ontario, there was another Kirwan family in the 1880s. Family lore makes no mention of this family; however, I would be curious to know more about them.
I do not know why Martin Kirwan moved to Sarnia (sometime between 1856 and 1860 based on baptism records found for sons Martin and Joseph). There is no family lore regarding other family members living in Lambton County at the time or reason given for the move. The Grand Trunk Railway reached Sarnia in 1856 when the area was experiencing an oil boom and this might have been the motivation to move there although this is pure conjecture on my part.
8.: Family lore has it that my great grandmother had come across the name Grandcourt in her reading, had taken a fancy to it and so it was that her first son was named Patrick Grandcourt. There was a somewhat similar occurrence in the naming of her second son, Vernon.