Buy your iPhone from the States or the UK instead and get it unlocked so it will work with any network here.
If you bought your iPhone package from O2 in Northern Ireland, you’d get 3 to 4 times as many minutes of use and 3 to 4 times as many texts for the same money you’d pay them here. This is a cynical rip-off by O2. Don’t stand for it.
Read this excellent post on how O2 are ripping off their Irish customers.