A quick Google will bring up a few options
I'd suggest you start with a trip to Blueface.ie
If you like the look of it send them an email describing what you want to do.
( I have been amazed at the quality and promptness of their responses - such a change from Eircom )
Of course you do not have to pick a voip provider based in Ireland, or indeed just one provider.
Also, if you hunt around you will find some providers - based abroad - that offer calls to Irish mobiles for 3-4c per minute. ( Ah - the joys of local regulation )
Olddog