Ar is a simple preposition that means on. It usually lenites except in certain set phrases, of which this is one.
Fad is a masculine noun that means length, distance, duration, extent.
Thus, ar fad: entirely, altogether, all ⁊rl…
Similar phrases, broadly interchangeable with ar fad, include go léir (particularly in Munster) and uile (Connacht).
In naturalistic speech be careful with the pronunciation of uile: