Noel is correct in that different people earning the same gross pay can end up paying a different amount of PRSI in the year. {Note that this still doesn't resolve the possible error in my Excel spreadsheet- that will be tested by me later tonight or tomorrow when I'm free}
For example, if your income is received in an even manner over the year at a rate of €6500 per month, you'll end up paying €2737 in PRSI (excluding Health Levy) on a grosss of €78000. If however, you received a trio of large lump sums (€26000 x 3 months) then you'll only end up paying €2014 in PRSI on the same gross- odd but true!
What's even crueller, however, is that a person whose annual salary is slightly below €26000 but then receives a payrise which brings them slightly above €26000 will actually see their net pay REDUCE due to the liability arising on the Health Levy..... incredibly unfair. Such a person has 3 possible options:
1. Refuse the pay increase- not a great option, there are better possibilities listed below
2. Pay enough into a pension scheme so that their annual income is brought below €26000 eg if I am earning €25,500 and then receive an increase to €26500, I'd pay 500+ per annum into a pension scheme to ensure that my 'reckonable income' is brought below the magic number of €26000. This way, you'll totally avoid the Health Levy, create a small pension asset and see your monthly income increase marginally. This appears to be the most profitable method.
3. Ask for the pay increase to be paid in a single lump-sum (pehaps in December for Christmas?). That way you'll only pay the Health Levy in that particular month but your net won't be as good as option 2.