"Cad and céard are equivalents. Cén is used before nouns. So cén fath really means what reason."
Who = cé, cé an duine (literally who is the person)
What = cé, ceard (short for cé an rud, I think), cad, cad é (also written caidé or goidé)
When = cén uair/lá/am, etc., cathain
Where = cá háit, cén áit
How = cén chaoi (literally cé an chaoi, what is the way), conas
Why = cén fáth (literally cé an fáth, what is the reason)