1. Taxable lump-sum is taxed at higher rate of tax i.e. 41% at the time of payout but you can claim a refund on it at the end of the year. Rate applicable is the average rate of tax for the last three years (relief is called top-slicing relief)
2. You can also claim a refund on PRSI at the end of the year by taking into consideration the earnings for the entire year.
3. Same with Income tax levy. Depending on your income for the full year, you would be able to determine which band is applicable to you. At the time of payout, PRSI and levy is charged at highest rate as the payroll process assumes this income to be your period income and assumes that it will be the same for remaining period of the year.