Phoning Ireland doesn’t work?

Yes, if its northern ireland, which is part of the UK, both Smarty and Voxi include calls to northern ireland in their plans, as will all UK networks of course, due to northern ireland being part of the UK.

If you are calling the Republic of Ireland, with both Voxi and Smarty, that's an international call, so credit or add ons must be used. Again, this will of course be the same with all UK networks.