I'm guessing that she may be paying no Income Tax at present either, in which case she would not be able to claim tax relief on her pension contributions, but will probably be taxed, at least in part, on her pension.
That said, if she expects to be in the tax net within a year or two, she could contribute to a PRSA and claim her tax relief at a later stage when she is paying tax. A PRSA would probably be the most suitable vehicle for small contributions as the charges are based on a small percentage of the contribution.
Jem certainly has a point, although a counter-point would be that if she starts the pension habit this early, it's likely to stay with her for life and will thank you for it when she's 40 and has accumulated a sizeable fund when others are scrabbling to start one.