253 certainly used to carry a blackthorn, but that was only because a certain CO carried one and if he did then everyone else had to as well. Whether or not they still do I don't know. Why not call in and ask?
Royal Irish Regiment ,London Irish Rifles ,Irish Guards officers all carry Blackthorns.
and the North Irish Horse sqn of the QOY (I think its a shorter version but may be wrong)
Maybe just check with the unit or wait until he joins the sqn ?
Nobody is going to shun him for not owning one when he turns up as a transferee....
Someone in my ken was once in an 'Irish' Battery (in fact, it might even have been called Irish Battery?) in the Gunners and he pretty soon was to be seen sporting black brogues and a Blackthorn in Barrack Dress!