Hi AMX3 that is an interesting query, I found an 1845 birth of a Mary McGennis mother Mary Mylott in St Catherines, 4th July, that is today too, coincidence. Now searched Roots Ireland and Irish Genealogy for a marriage and no go but was using McGuinness. Name variants should take care of it though. It is possible that this child died.
Around this time in North Dublin a Christopher McGennis and a Mary Caulfield (1840 marriage) had a number of children including twins but no mention of Mylott, another odd thing I noticed was in searching the free site Irish Genealogy where I got the above 1845 birth, is the number of Marrige hits I got for Christopher McGuinness and variants but he is the witness in an awful lot of marriages, more than he should be in my opinion, thats all it is is an opinion, the site is here, select church and Dublin and search, very few church records on it but excellent for Dublin. https://www.irishgenealogy.ie/en/
I will search or ask on the Ireland XO FB page to see if this number of witnesses of that name is usual or if it is just chance. I cannot see your Christipher on the above site, really odd.