I'm struggling also

You've gone from looking for William Montgomery & wife Mary McKeown parents of an unknown daughter who married in Belfast in 1874 to "Equally, I have a strong suscipion that the information provided by the bride at her baptism & marriage could be false and not just that her father may not be William, but also that her mother may not be Mary."
When we ask questions there are good reasons for this. If we had the daughter's details to establish her age then we might have a better idea of when her parents might have been born. If daughter was married in Belfast there's a chance that siblings might also have done so even if family might, or might not, have always lived in Belfast.
However, as Elwyn says, parents could have died before civil registration of deaths, may have lived outside Belfast, etc.