Is this the exact amount I will receive in payroll?
No. This is an estimate for planning. Payroll basis, credits applied, pension setup, and taxable benefits can change the real outcome.
Why can two people on the same salary see different take-home pay?
Credits, USC/PRSI reliefs, pension arrangements, benefit-in-kind, and payroll timing can all change the result.
Why can the marginal deduction on extra income look high at higher salaries?
Once additional income falls into higher-rate bands, the combined effect of PAYE, USC, and PRSI can make the marginal deduction look high. The exact outcome depends on your inputs.