If you mean refusing a pension, I do not think it has any benefit because the person is forgoing the income.
If you mean accepting the pension and donating it to charity then the person would have say €10,000 of PAYE Income subject to PAYE, PRSI, Health Levy, Income Levy, (I'm open to correction on the rates applicable to state employees pensions)
Then they would get a deduction against their PAYE Income so they would end up the same if not a little worse off due to PRSI ect.