After working for twenty years I can roughly estimate my pay after tax but can't can't do the sums to the exact euro. At this stage I should have a better idea of how to do the calculations.
Starting from the basic and assuming a childless single person without any other deductions:
Gross pay:40k
Personal tax credit: 1650
Employee tax credit: 1650
Flat rate expenses: 0
Medical insurance relief: 120
Can anyone show me how to calculate my net monthly pay?
I've a spreadsheet available at http://taxcalc.eu/monthlyss but the detailed calculations (as opposed to the actual results of those calculations) aren't visible so it probably isn't what you're seeking.
Apart from items mentioned above, you would also need to allow for USC and PRSI. PRSI is 4% of gross pay for most employees but there are a few exceptions and also certain deductions to take into account.
Details of rates and thresholds for USC in 2018 are available [broken link removed]
Here is a little Excel calculator I created for myself and changed to input your info. You can play around with additional deductions / BIK / Pension. E&OE.