I dont see any sign of Catherine & Michael as a couple, or baptisms for the children you mentioned (Jane: c1841, John: c1843, Susan: c1847). Maybe they were born in a parish that's not on the system yet (several large RC parishes have yet to be added - St. Mary's Marlboro St, St. Pauls Arran quay etc) Maybe they lived outside the city or county ?
Is there any mention of where they lived (or clues to the area) in Micheal's letter ?
I also cannot locate any Canavan (or similar) baptisms in St. George's parish or Dignam's Lane... to confirm if they are children of Michael & Catherine or not. Maybe these three children died when they were too young and missed that. Ages on burial records were 9 Dys, 5 Wks, and 9 Dys. Some of my CofI ancestors were baptised months after birth, whereas RC baptisms seemed to generally take place more quickly - sometimes even on the day of birth.
Shane