Not enough detail to say if you have to pay Prsi. There is a PRSI free allowance per pay period ( €127 per week etc. non-cumulative), also an earnings threshold (€352 per week in 2009) below which no Prsi is paid.
Medical card holders and persons over 70 are also exempt from Prsi.
If you do have to pay Prsi and health levy, it would be on the gross interest received ( before DIRT).