I wouldn't think of it as a numeral, especially as numbers typically go before the noun, rather than after it. ar fad can modify a noun, in which case it usually means "the whole" for a singular noun, or "all" for a plural noun, so an lá ar fad is "the whole day" (or "all day long", to tie in with the "length meaning of fad), an scéal ar fad - "the whole story", na daoine ar fad - "all the people".
When ar fad qualifies an adjective, it's usually an intensifier like "really", "completely" or "totally". iontach ar fad - "really wonderful", sin scéal eile ar fad - "that's a different story entirely", Tá said go haoibhinn ar fad - "they are absolutely gorgeous" etc.
ar fad is an idiom, and the translation isn't always going to be exact, so the NEID for example, translates "the house was completely destroyed" as scriosadh an teach ar fad, whereas I would usually back-translate that as "the whole house was destroyed", but idiomatically, there's very little difference between "the house was completely destroyed" and "the whole house was destroyed" in English. But the ar fad in this exercise is clearly qualifying the noun na páistí, not the verb, so "all the children" is the intended meaning.