Who is allowed to "reasonably chastise" someone though? Is that just parents? It could be because this guy was a stepfather and not the girl's real father. I haven't seen the report - "hitting" is a bit of a catch all phrase, was it a gentle slap on the legs (isn't that what parenting books talk about?) or did he belt her around the room.
As a matter of interest, this is all I found after a quick search on Oasis, which only seems to have information on discipline in schools:
Corporal Punishment [in schools]
Under the Non-Fatal Offences Against the Person Act, 1997, corporal punishment of students may be a crime.
However, the Act permits the justifiable use of force in certain circumstances, e.g., self defence or defence of others.