Provided you earn over €400 a week -- you pay a Health Levy of 2% on the total amount earned, otherwise you pay no health levy (I think.). After that, Class A1 PRSI is also applied at 4%. The first €551 you earn a month (€127 a week) is exempt from PRSI and not included.
So, take 2% of your total income for the period (before tax). This is your Health Levy (assuming your income is over €400 a week.).
Subtract €551 from your total income for the period. Take 4% of the remainder -- this is your PRSI.
Add Health Levy to PRSI to get your total "PRSI".
In your case -- perhaps your PRSI is being calculated on a weekly basis even though you are paid monthly? So rather than €551 being subtracted from your income, €127 is multiplied by the number of weeks in the pay period. That seems slightly unorthodox, but would explain how your PRSI could vary from month to month, even though your pay before tax doesn't.
PRSI is the most complicated part of the "tax" system. This is how my payslip works, and I guess most other PAYE workers. It's also possible (but unlikely) that you fall into a different PRSI class so the rate or exemption limits may be different.
A related complication is you only pay the 4% PRSI on the first €46,600 (before tax) you earn in 2006. For some people this means their PRSI decreases in later months!