Having previously been employed in Ireland:
If you had a previous employment in Ireland within the current tax year you must submit your current P45 to the payroll department. This P45 is then returned to the revenue. The revenue will process this P45 and ensure that all the figures are correct.
An updated tax cert , generally on a normal (cumulative) basis, will then be issued by the revenue and any overpayment of tax, if any, will be refunded to you in the period a normal tax cert is processed.
Having NOT previously been employed in Ireland:
If you have not had previous employment in Ireland you must contact the revenue, quote your own PPS number and the registered number for your employer. Once the revenue has ascertained the length of employment within Ireland an updated tax cert, generally on a normal basis will be issued.
Once a tax cert on a normal basis is processed through the payroll any overpayment of taxes, if any, will be refunded to you in that period.