PRSI Contribution period

I have read on the welfare.ie site that a PRSI contribution period is one week. However i have changed jobs in 2009 and my new employer's payroll company have told me that the PRSI contribution period is 4 weeks if i get paid monthly.
So according to them , if i started working with them on say 27 November then i have to pay 4 weeks PRSI even though i would only have worked a few days in that month. This sounds wrong to me , i thought i should only pay PRSI for the actual number of weeks i work. Does anyone know if my company are correct in what they say ?

PRSI is deduced as a percentage of your salary, so what you are saying makes no sense. They are not going to deduct 4 weeks PRSI from you, when tou have not worked a full month as there is no such thing.

If you only work a few days, and only get paid accordingly, PRSI will be deducted as a percentage of the small payment you get.

You have obviously misunderstood what they mean.

Te only reason that the weekly / monthly issue makes a difference with PRSI is that
if you earn 127 or less a week, then you dont pay PRSI.

If you are paid monthly, they pro-rata this to 550 a month.

Sounds like someone is going back to the old days of physically buying stamps and putting them on a card and the rules which then applied. Anyway, what you are talking about are insurable weeks, most companies who pay monthly would just give 4 or 5 for the month and not bother to change them (or know any better) if you started/left during a month. They don't cost money, but are just a number to indicate the number of weeks you are employed there.
They are subsequently used when calculating your state benefits.
See :http://www.welfare.ie/EN/Publications/SW3/Pages/42Weeksofinsurableemployment.aspx