If she leaves during the tax year then any unused tax credit/cut off point can be claimed back.
If she leave employment in Ireland in say June her situation could be as follows:
I am making the following assumptions
Annual Salary 35,000
Cut off point 31,000
Tax Credit 4,000
After 6 months
Situation as follows
Salary 17,500
Cut Off point 15,500
Tax @ 20% 3,100
Tax @ 42% 840
Total Tax 3,940
Tax Credits 2,000
Tax paid 1,940
Situation at end of year
Salary still 17,500
Cut off point 31,000
Tax @ 20% on 17,500 = 3,500
Tax credits 4,000
Tax due Nil
Tax refund 1,940
Don't know if PRSI can be reclaimed
Revenue will allow this tax to be repaid quickly within a few weeks quicker if you hassle/ask them nicely on completion of a form and a note declaration (may be part of the form) stating that you will not be working in Ireland for the rest of the tax year i.e. to 31/12/06.