I would have thought you would have to swap your calls to BT too. eircom owns most of the lines in Ireland. BT pay eircom the line rental (@ wholesale rate) so I couldn't see them being to happy to to take you on as a customer for a couple of euros a month. It would probably cost them that in admin costs.
You will pay the line rental no matter who you go with. Some companies bundle it into their call packages ie BT have an offer at the moment €45 a month for line rental, broadband, and unlimited national landline and UK calls. Some firms offer line rental a couple of euro cheaper (€20/21) but you have to give them your call revenue too.