Was her only income PAYE and when is she returning to Ireland?
If someone is a PAYE employee and won't be resident for the year following the year of departure then they are entitled to "split year treatment" for the year of departure, which means that they get the full benefit of their unused tax credits for that year. If the person doesn't meet the residency criteria for the following year then they cannot avail of this relief.
Also, the relief relates to PAYE employment only, so if a person has self-assessed income it won't apply. If this is the case then the only tax credits that are due are for the portion of their worldwide income that relates to the Irish income, i.e., if 1/3 of their total worldwide income for the year is from here, they get 1/3 of the tax credits.
The Revenue website has more info about it.