I'm understanding you have access to ACOM public member trees, correct?
I did some research on this line and put the data in a public research tree, you can go here (Jean Baptiste/John Gagnon's (b abt 1816-1825) profile) and view the data in context:
http://trees.ancestry.com/tree/29249304/person/26223232884I see many, many challenges with this difficult search; even with all the resource and advance research skills, there are no simple answers due to the following:
1) Inconsistent census data, ie; significant variations in the birth years and birth locations and faith practiced. As a general practice, the data in earlier census tends to be closer to the verifiable facts. I've not concluded this is true in the case.
2) This particular geographical area as well as the time period, presents many more challenges because the documentation can be sparse, incomplete or non-existent indexing, and/or illegible registers.
3) I found NO associated family (both GAGNON or FOGARTY) in ANY of the bap records of the couples' children. This indicates to me, the couple may have married elsewhere prior to migrating and settling in the Chichester, Pontiac region. I have found no marriage record for the couple in the Quebec resources, however that doesn't mean one doesn't exist, the indexed sources I'm using are biased toward Catholic records. The first record I found for this line, is for the (apparent) first born child, Edward (b 1 Dec 1850, baped 19 Jan 1851, Chapeau (St-Alphonse). If he was indeed the first born, it can be asserted his parents were married in the early months of 1850. There is no marriage record in Chapeau (St-Alphonse), the register commences in 1846.
There is the possibility they were not married in the Catholic Church as Mary Ann Fogarty is cited as "Protestant" in the 1851 census, as is her son Edward (b 1850), however Edward was baptized in the Catholic Church (19 Jan 1851), the bap document verifies this.
4) I found NO other Fogarty in the Chapeau/Pontiac region in the 1851 Census. Marie Anne Fogarty is with her son Edward in the 1851 Census, but John Gagnon in not with them. As best as I decipher the data in the 1851 Census, Mary Anne and son Edward, were in the household of a Joseph Lepine and Mary Egan, who also stood at the godparents to Edward Gagnon (b 1850). The household below Mary Anne Fogarty and son Edward Gagnon is that of (Jean) Baptiste Carrol, who would become the father-in-law of Edward Gagnon. Thi
5) My standard practice of finding where a marriage might have occurred, is to follow the bride. The 1842 and 1851 Census should be combed for Fogarty/Fogharty. This is to determine locations of where to look for more data. I've been attempting this with very minor results.
6) Naming patterns were common during this period, and should be considered. Looking at the given names of the ALL the children, may reveal some clues and provide a direction to take. It appears that this couples' #7 & #8 children were the namesakes of their parents (son John, b 1861, daughter Marie/Mary Ann, b 1863). There are 5-6 children born before John and Mary Ann, and it is highly likely that at least a few of them were namesakes for associated family. Some of the given names could be considered to have origin in either the maternal or paternal line, i.e.; Edward, Joseph, Francis. Other given names, such as Richard, Sara Jane, Angus are decidedly from the maternal line.
You should note I'm building on the naming pattern in an attempt to determine origin of the Fogarthy line, there are very distinct patterns over 3 generations I'm looking at in Quebec. I'm finding Fogarty's marrying in the Protestant Church and baptizing their children in the Catholic Church. Please note the disclaimers I've included on the profiles.
7) There is another male (adult), Joseph Gagnon living in the Chapeau area in the same period. I would reconstruct his line and see if there's some correlation to your Jean Baptiste/ John Gagnon.
8) There are 59 "Jean Baptiste/Jean Gagnon's" born in Quebec btw 1816 and 1825. This makes the option of reconstructing each and everyone of them a monumental task. Discounting who it can't be is a proven method to winnow down to who it could be. This works well if you have less common names, which is not the case here, it all depends on how much time you're willing to devote to go down roads that may dead-end or prove they're not the line you're looking for.
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I looked at your tree data, and am curious why you cite Jean Baptiste Gagnon (b abt 1816-1825, every census has varied birth years in a 9 year span) as being born in Normandy, France, and Marie/Mary Ann Fogarty/Fogherty, (b abt 1828-1832) born in England? I'm not finding hard data to support those assertions, but you may have some story passed down the line? The data I've evaluated makes the birth origin location of both, most likely, the Province of Quebec, Canada.