I haven't yet found the marriage of Andrew and Ann, but I believe I see the family at West Zorra, Oxford County, Ontario, in the 1851 census (actually taken in 1852). Andrew is 30, a farmer, born Ireland. Ann P. is 22, born Ireland. Their children are Matthew, 3, born Canada West, and John, 1, born Canada West. In 1861, I see this family at Greenock, Bruce County. Chepstow, the place where Matthew died, is in Greenock. Andrew is 40, Ann (listed as 33) appears to have drowned in 1860? or maybe this is a reference to Matthew's death?, John is 10...plus there are two more children, Lucy, 8, and Andrew, 6. The family is Roman Catholic. You may have seen these records already but I thought I'd mention them....just in case you haven't. I don't have a current subscription to Ancestry, but found the images of the censuses at http://www.bac-lac.gc.ca/eng/census/Pages/census.aspx
I'll keep looking for the marriage and will let you know if I find it.
Do you have a more exact location for Matthew's baptism record...perhaps a city or township? It's possible that the marriage took place there, as well.