Scroll through this page to get an idea of prices available.
Be aware that some companies charge a call set up fee, and then charge per minute on top.
Others do not have a set up fee but charge more per minute.
Most of my calls are very short, so I use a provider with no set up fee.
You may have a different calling pattern.
There are also apps to install on smart phones which will allow the user to use their wireless connection to call VOIP to VOIP for no charge.
There are lots of options, including getting an Irish number for your VOIP if that might be useful.
You have a bit of research to do