"They have much more water."
Translation:Tá i bhfad níos mó uisce acu.
It comes before the fad.
If you're asking why Irish uses i bhfad, it's pretty much the same reason that English uses "at length" in "He spoke at length on the matter". Prepositions don't always translate directly, or have a specific meaning, so we say "In honour of someone" but "out of respect for someone", even though "honour" and "respect" play basically the same role in those phrases.
English doesn't bother with a preposition before "far" when it is used as an intensifier ("much more" = "far more"), Irish does use a preposition i bhfad.
Note that i bhfad doesn't mean "in length" - that's ar fad or ar a fhad.