If you are comparing your PRSI deductions on a December payslip to a January payslip then the usual reason for the the increase is as follows
The PRSI contributions are made up of a 4% PRSI element and a 2% health levy when your accumulated pay for 2008 exceeded 50700 you no longer paid the 4% PRSI element and only pay the 2% levy.
When you start a new tax year as your accumulated pay is below 52000( for 2009) you pay the 4% PRSI element again and the 2% health levy.
If you comapred a January 2008 payslip to a January 2009 payslip there should be very little change in PRSI deductions if your salary has not increased alot