I searched for a marriage between William Wilson & Anne Jane Irwin in Fermanagh 1850 – 1870 but did not find one on rootsireland or in the civil marriage indexes (which start in 1845 for non RC marriages and 1864 for RC). Nor did I find a William Wilson baptism in Fermanagh c 1831.
The Church of Ireland baptism records for Killesher start in 1798. I don’t think they are on-line anywhere but there is a copy in PRONI, (the public record office) in Belfast. A personal visit is required to view them though.
Photo of St John’s here: https://www.flickr.com/photos/16132340@N07/5563975973
Wilson is a very common surname in Fermanagh with 569 of them in the county in 1901, of whom 41 were named William Wilson. So the name would have been equally common in the 1830s and you really need to try and get William’s parents names from his death certificate or other source to be sure of identifying the right family. Or, if you know what townland the family lived in, that might help too.