You're not missing anything, there are points along the PRSI scale where one can have a greater net pay for a lesser gross pay. This arises where the deduction , levy or whatever, applies on all the gross pay, not just that over the threshold being exceeded. At €289 pw no I/Levy applies. At €290 pw 2% applies on all resulting in a deduction of €5.80. This happens similarly with the Health Contribution on the €500 threshold as you mentioned, it's just the way the calculation works.