One of his son's name, if not the same as his own will be for his father, usually the first son - John in this case? The 2nd. son would be after her father.
Robert and McFadden are very Scot's. Fermanagh records are
very late but you can look for his own birth/baptism in
an Irish History Foundation, but very few Parish's had
records this far back?
Usually a marriage record anywhere would have his father's name at least, only a birth record in Ireland would have
both mother and father's name.
Were they Methodist? or Church of Ireland (England)
There were no McFadden's in the Tithe Applotments for
Fermanagh. Enniskillen, Fermanagh was also a military
establishment and these soldiers would not of occupied land.
By 1862 there are 53 McFadden's are listed in the Griffith Valuation (census substitute similar to the Tithe's)There
are 12 named John. John is the most used name in Ireland, followed by James and Patrick. Unlikely his father would of been alive by then but may have had brothers and sisters who would all have one child in each family named after their father.
If your family were Church of Ireland or Methodist the
parish registers that do exist maybe held at the
Church of Ireland, Representative Library in Dublin up to 1899. Visits are in person only.
ps:I have Church of Ireland marriages 1845-1862