For permanent there are certainly deals to be had, albeit it may depend on the type of people you are looking for and the numbers/salary involved, but 10-12% would be the most I'd expect to pay and unless it was a technical/specialist role, I wouldn't look at an agency charging 15%.
For temps, it's more complicated, I'm assuming the agency picks up employers PRSI, but are they making any payments relating to pensions and is leave paid/unpaid and if paid, who's paying, are you paying the agency directly for the days in question or is it included in their mark up?
Remember VAT as well, hence for perms, it may be cheaper to try and do it yourself