The very worst case would see you paying the following deductions (* = see notes below):
PAYE 41%
Highest public sector pension levy 10.5% *
Highest income levy 6% **
Your pension 3%
PRSI 4%
Health contribution 5% ***
Total deductions 69.5% of the overtime, leaving you with 30.5%
See the following notes- basically to be hit as hard as this you'd need to be earning monthly somewhere north of €14582.... which at the overtime rate you indicate seems unlikely.
Also, if your tax credits were reduced to 0 for the month it might be possible that your PAYE has increased?
Overall, it seems a VERY high deduction.
Notes:
* If your monthly earnings exceed €6000 and you are employed in the public/civil service
** If your monthly earnings exceed €14582 (lucky you!)
*** If your monthly earnings exceed €6253