I may be able to offer some bits of info, but it's all speculative.
My great-grandfather, James Cullen, was born in that same parish (Philipstown/Daingean) in 1836, the same year as that marriage, and later in 1869 a Kate Fox was a sponsor at the baptism of one of his nephews, so the Fox's may have been cousins of the Cullen's. James had left for America before that baptism (he fought in the US Civil War). The Cullen family lived in the townland of Eskermore, which can still be seen on Google Earth along the road to the east of Daingean, so perhaps the Fox family lived nearby. I've been to Ireland several times, but not to Daingean, since I didn't have this info the last time I was there with a car. I'm on GEDMATCH as well (T780556), in case you want to compare results.
I assume that you've already searched the online parish registers, so you know that the Philipstown/Daingean records go back to 1798, but with a gap from 1799-1819 as to baptisms, which is unfortunately the time period you likely need for the baptisms of Christopher and Margaret. I have all of the pages downloaded, so I checked out the marriage record you already have, to see whether there was any other info to be gleaned (sometimes cousins were married on the same day), but no luck (I was lucky that my ancestors fell on either side of the gap, so I was able to find the baptismal record for James's father, John, and thus the names of John's parents, too).
I know from other branches of my family that people often married very young back then (especially brides), so since I had the records, I checked the years 1820-21 in the baptismal records, in case Christopher or Margaret were baptized then. No luck, but I noticed that a Charles (Carolus) Byrne was baptized on 24 June 1821, his parents being John (Joannes) Byrne and Mary (Maria) Dunne (image 24 on film 04202/09) . Unfortunately, the priest did not indicate the townland where they resided (some other parish records do that).
Since the wedding record you have is from that parish, and weddings usually took place in the bride's parish, that Charles may have been a younger brother of your Margaret (and John and Mary thus the parents of your Margaret). There was also a baptism of a Denis (Dionysius) Dunne on the same page, and a Diarmuid (Demetrius) Dunne later that year (on another image). The Dunne's seem to be fairly numerous, but not the Byrne's, which may make it even likelier that Charles Byrne was your relative. Interestingly, there were also a Brigid Cullen and a Margaret Roarke baptized in 1821, who I believe were second cousins of my James Cullen.
That's about it (I did say this is all speculative), but you might go through the baptismal and marriage registers for the years when Christopher and Margaret were older, to see whether they acted as witnessses at weddings or sponsors at baptisms, and see whether they link with other Dunne's.
