The basic rules are that you can earn €2,666.66 per month at the lower tax rate (20%) with the balance taxed at 42%. The total of these 2 tax amounts is then reduced by your monthly tax credit which is €260.
That explains the PAYE part of your tax deductions and should be quite easy to reconcile to your payslip.
Separately there are 2 prsi deductions. The first is the health levy which is 2% of total income. The 2nd is prsi which is 4% however the first €550.33 of income per month is prsi-free. There is also a ceiling above which no prsi is payable – that is €39,994 for 2006 (€46,600 less 12 monthly exempt amounts of €550.33).
I can’t reconcile the figures you quote to the above but if you post the exact details from your latest payslip it should be possible to see if anything is going wrong.
At a quick glance I don't know if you’re overpaying tax. Your post is from 19/11/06 so the figures you quote, assuming they are up to and including October 06, show that you’ve earned about 33% more than you did last year. That obviously increases significantly your tax bill because every extra Euro you’ve earned has been taxed at 47%.
Your PAYE bill in 2005 should have been €3,912. That is ignoring the health levy and prsi.