Is my friend legally entitled to have her deposit refunded in the
following situation? (In the Republic of Ireland)

My friend viewed a car at a car dealers.  She liked the car, and put a
booking deposit on it in cash (I assume there was no mention made of
refundability at that point).  The very next day, she found a
different car elsewhere that she liked better, and so she went back to
the dealer to get her deposit back because she had changed her mind. 
The dealer refused to refund the deposit.