If your girlfriend gets full pay while on maternity leave and the company has her pay over her maternity benefit payments then make sure that she doesn't end up effectively paying tax on MB! This issue is covered elsewhere if I'm not mistaken.
Also - especially if she does not return to work this year and depending on her maternity leave salary arrangements - she should make sure to request a P21 balancing statement from Revenue at the year end to make sure that she has not overpaid tax.
As ever you/she should make sure that you are claiming all relevant credits/allowances. See they key posts.