As PRSI is calculated on a 'week one' basis, Payback won't make any automatic adjustments to YTD PRSI contribution (if the class was entered incorrectly for example).
If the incorrect PRSI class was entered on the employee screen but the pay or P30 for that pay period hasn't been issued yet then; rewind the payment in the payroll screen, select the correct PRSI class on the employee screen, and then re-process the payment on the payroll screen.
Refunding PRSI payments is the responsibility of Dept of Social and Family Affairs. See this page for more info on obtaining a refund:
http://www.welfare.ie/EN/OperationalGui ... funds.aspx